diff --git a/src/Oms.Application/Dtos/Orders/OrderDownstreamForwardDtos.cs b/src/Oms.Application/Dtos/Orders/OrderDownstreamForwardDtos.cs
new file mode 100644
index 0000000..ecd872d
--- /dev/null
+++ b/src/Oms.Application/Dtos/Orders/OrderDownstreamForwardDtos.cs
@@ -0,0 +1,35 @@
+using Oms.Application.Dtos.Orders;
+
+namespace Oms.Application.Dtos.Orders;
+
+/// <summary>
+/// 下游 createOrderOne 请求体预览（由订单表组装，不含 AppSecret）。
+/// </summary>
+public class OrderDownstreamCreateOneBodyOutput
+{
+    public int OrderId { get; set; }
+
+    public string? ServiceNo { get; set; }
+
+    public string SupplierCode { get; set; } = string.Empty;
+
+    public OrderParcelCreateInput Body { get; set; } = new();
+}
+
+/// <summary>
+/// 下游 createOrderOne 转发结果。
+/// </summary>
+public class OrderDownstreamForwardResult
+{
+    public bool Success { get; set; }
+
+    public string SupplierCode { get; set; } = string.Empty;
+
+    public string Url { get; set; } = string.Empty;
+
+    public string RequestJson { get; set; } = string.Empty;
+
+    public int? StatusCode { get; set; }
+
+    public string ResponseBody { get; set; } = string.Empty;
+}
diff --git a/src/Oms.Application/Integrations/Itfx/IItfxForwardClient.cs b/src/Oms.Application/Integrations/Itfx/IItfxForwardClient.cs
new file mode 100644
index 0000000..c5a5ce8
--- /dev/null
+++ b/src/Oms.Application/Integrations/Itfx/IItfxForwardClient.cs
@@ -0,0 +1,24 @@
+namespace Oms.Application.Integrations.Itfx;
+
+public sealed class ItfxForwardResult
+{
+    public bool Success { get; set; }
+
+    public string Url { get; set; } = string.Empty;
+
+    public string RequestJson { get; set; } = string.Empty;
+
+    public int? StatusCode { get; set; }
+
+    public string ResponseBody { get; set; } = string.Empty;
+}
+
+public interface IItfxForwardClient
+{
+    Task<ItfxForwardResult> ForwardCreateOrderOneAsync(
+        string supplierCode,
+        string appKey,
+        string appSecret,
+        object body,
+        CancellationToken cancellationToken = default);
+}
diff --git a/src/Oms.Application/Integrations/Itfx/IItfxMetadataClient.cs b/src/Oms.Application/Integrations/Itfx/IItfxMetadataClient.cs
index c960b1f..427b03a 100644
--- a/src/Oms.Application/Integrations/Itfx/IItfxMetadataClient.cs
+++ b/src/Oms.Application/Integrations/Itfx/IItfxMetadataClient.cs
@@ -3,6 +3,8 @@ namespace Oms.Application.Integrations.Itfx;
 public sealed class ItfxAppMetadata
 {
     public string AppKey { get; set; } = string.Empty;
+
+    public string AppSecret { get; set; } = string.Empty;
 }
 
 public sealed class ItfxServiceProviderMetadata
diff --git a/src/Oms.Application/Integrations/Itfx/ItfxForwardClient.cs b/src/Oms.Application/Integrations/Itfx/ItfxForwardClient.cs
new file mode 100644
index 0000000..196f874
--- /dev/null
+++ b/src/Oms.Application/Integrations/Itfx/ItfxForwardClient.cs
@@ -0,0 +1,72 @@
+using System.Net.Http.Headers;
+using System.Net.Http.Json;
+using System.Text.Json;
+using Fake.DependencyInjection;
+using Microsoft.AspNetCore.Http;
+using Microsoft.Extensions.Options;
+
+namespace Oms.Application.Integrations.Itfx;
+
+internal sealed class ItfxForwardClient(
+    IHttpClientFactory httpClientFactory,
+    IOptions<ItfxGatewayOptions> options,
+    IHttpContextAccessor httpContextAccessor) : IItfxForwardClient, IScopedDependency
+{
+    public const string HttpClientName = nameof(ItfxForwardClient);
+    public const string CreateOrderOneStandardCode = "createOrderOne";
+
+    private static readonly JsonSerializerOptions JsonOptions = new()
+    {
+        PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
+        WriteIndented = true
+    };
+
+    public async Task<ItfxForwardResult> ForwardCreateOrderOneAsync(
+        string supplierCode,
+        string appKey,
+        string appSecret,
+        object body,
+        CancellationToken cancellationToken = default)
+    {
+        var settings = options.Value;
+        if (string.IsNullOrWhiteSpace(settings.BaseUrl))
+        {
+            throw new InvalidOperationException("Itfx BaseUrl not configured");
+        }
+
+        var root = $"{settings.BaseUrl.Trim().TrimEnd('/')}/InterfaceForward/ForwardWithSubscribe";
+        var url =
+            $"{root}?standardCode={CreateOrderOneStandardCode}&supplierCode={Uri.EscapeDataString(supplierCode)}";
+        var requestJson = JsonSerializer.Serialize(body, JsonOptions);
+
+        using var request = new HttpRequestMessage(HttpMethod.Post, url);
+        request.Headers.TryAddWithoutValidation("appkey", appKey);
+        request.Headers.TryAddWithoutValidation("appSecret", appSecret);
+        request.Headers.TryAddWithoutValidation("followName", "0");
+        ForwardAuthorization(request);
+        request.Content = JsonContent.Create(body, options: JsonOptions);
+
+        var client = httpClientFactory.CreateClient(HttpClientName);
+        client.Timeout = TimeSpan.FromSeconds(Math.Max(30, settings.TimeoutSeconds));
+
+        using var response = await client.SendAsync(request, cancellationToken);
+        var responseBody = await response.Content.ReadAsStringAsync(cancellationToken);
+        return new ItfxForwardResult
+        {
+            Success = response.IsSuccessStatusCode,
+            Url = url,
+            RequestJson = requestJson,
+            StatusCode = (int)response.StatusCode,
+            ResponseBody = responseBody
+        };
+    }
+
+    private void ForwardAuthorization(HttpRequestMessage request)
+    {
+        var authorization = httpContextAccessor.HttpContext?.Request.Headers.Authorization.ToString();
+        if (!string.IsNullOrWhiteSpace(authorization))
+        {
+            request.Headers.Authorization = AuthenticationHeaderValue.Parse(authorization);
+        }
+    }
+}
diff --git a/src/Oms.Application/Integrations/Itfx/ItfxMetadataClient.cs b/src/Oms.Application/Integrations/Itfx/ItfxMetadataClient.cs
index 79aaf7b..36b0b83 100644
--- a/src/Oms.Application/Integrations/Itfx/ItfxMetadataClient.cs
+++ b/src/Oms.Application/Integrations/Itfx/ItfxMetadataClient.cs
@@ -24,9 +24,17 @@ internal sealed class ItfxMetadataClient(
         GetAsync<ItfxAppMetadata>($"/App/GetInfo?id={appId}", payload =>
         {
             var appKey = ReadString(payload, "appKey", "AppKey");
-            return string.IsNullOrWhiteSpace(appKey)
-                ? null
-                : new ItfxAppMetadata { AppKey = appKey.Trim() };
+            var appSecret = ReadString(payload, "appSecret", "AppSecret", "secret", "Secret");
+            if (string.IsNullOrWhiteSpace(appKey))
+            {
+                return null;
+            }
+
+            return new ItfxAppMetadata
+            {
+                AppKey = appKey.Trim(),
+                AppSecret = appSecret?.Trim() ?? string.Empty
+            };
         }, cancellationToken);
 
     public Task<ItfxServiceProviderMetadata?> GetServiceProviderAsync(
diff --git a/src/Oms.Application/Integrations/Itfx/ItfxServiceExtensions.cs b/src/Oms.Application/Integrations/Itfx/ItfxServiceExtensions.cs
index f326f3d..ffe6558 100644
--- a/src/Oms.Application/Integrations/Itfx/ItfxServiceExtensions.cs
+++ b/src/Oms.Application/Integrations/Itfx/ItfxServiceExtensions.cs
@@ -9,7 +9,9 @@ public static class ItfxServiceExtensions
     {
         services.Configure<ItfxGatewayOptions>(configuration.GetSection(ItfxGatewayOptions.SectionName));
         services.AddHttpClient(ItfxMetadataClient.HttpClientName);
+        services.AddHttpClient(ItfxForwardClient.HttpClientName);
         services.AddScoped<IItfxMetadataClient, ItfxMetadataClient>();
+        services.AddScoped<IItfxForwardClient, ItfxForwardClient>();
         return services;
     }
 }
diff --git a/src/Oms.Application/Services/Orders/OrderService.DownstreamForward.cs b/src/Oms.Application/Services/Orders/OrderService.DownstreamForward.cs
new file mode 100644
index 0000000..ed1d862
--- /dev/null
+++ b/src/Oms.Application/Services/Orders/OrderService.DownstreamForward.cs
@@ -0,0 +1,85 @@
+using Fake.Application;
+using Fake.Domain.Exceptions;
+using Microsoft.AspNetCore.Mvc;
+using Oms.Application.Dtos.Orders;
+using Oms.Application.Integrations.Itfx;
+using Oms.Application.Shared;
+
+namespace Oms.Application.Services.Orders;
+
+public partial class OrderService
+{
+    private OrderDownstreamCredentialResolver OrderDownstreamCredentialResolver =>
+        LazyServiceProvider.GetRequiredService<OrderDownstreamCredentialResolver>();
+
+    private IItfxForwardClient ItfxForwardClient =>
+        LazyServiceProvider.GetRequiredService<IItfxForwardClient>();
+
+    /// <summary>
+    /// 按订单 Id 从数据库组装 createOrderOne 请求体（预览，不调用下游）。
+    /// </summary>
+    [HttpGet("/order/downstream-create-one-body")]
+    public async Task<OrderDownstreamCreateOneBodyOutput> GetDownstreamCreateOneBodyAsync([FromQuery] int id)
+    {
+        var (order, body) = await LoadDownstreamCreateOneContextAsync(id);
+        var creds = await OrderDownstreamCredentialResolver.ResolveAsync(order);
+        return new OrderDownstreamCreateOneBodyOutput
+        {
+            OrderId = order.Id,
+            ServiceNo = order.ServiceNo,
+            SupplierCode = creds.SupplierCode,
+            Body = body
+        };
+    }
+
+    /// <summary>
+    /// 按订单 Id 组装 createOrderOne 并转发至 itfx 下游。
+    /// </summary>
+    [HttpPost("/order/downstream-forward-create-one")]
+    public async Task<OrderDownstreamForwardResult> DownstreamForwardCreateOneAsync([FromBody] OrderActionInput input)
+    {
+        var (order, body) = await LoadDownstreamCreateOneContextAsync(input.Id);
+        if (string.IsNullOrWhiteSpace(order.ServiceNo))
+        {
+            throw new BusinessException("订单运单号为空，无法预报下游");
+        }
+
+        var creds = await OrderDownstreamCredentialResolver.ResolveAsync(order);
+        var forward = await ItfxForwardClient.ForwardCreateOrderOneAsync(
+            creds.SupplierCode,
+            creds.AppKey,
+            creds.AppSecret,
+            body);
+
+        return new OrderDownstreamForwardResult
+        {
+            Success = forward.Success,
+            SupplierCode = creds.SupplierCode,
+            Url = forward.Url,
+            RequestJson = forward.RequestJson,
+            StatusCode = forward.StatusCode,
+            ResponseBody = forward.ResponseBody
+        };
+    }
+
+    private async Task<(Domain.OrdersAggregate.Entities.Order order, OrderParcelCreateInput body)> LoadDownstreamCreateOneContextAsync(
+        int orderId)
+    {
+        var order = await GetOrderOrThrowAsync(orderId);
+        var orderIds = new[] { order.Id };
+        var senders = await OrderRepository.GetSendersByOrderIdsAsync(orderIds);
+        var recipients = await OrderRepository.GetRecipientsByOrderIdsAsync(orderIds);
+        var declarations = await OrderRepository.GetDeclarationsByOrderIdsAsync(orderIds);
+        var weights = await OrderRepository.GetWeightsByOrderIdsAsync(orderIds);
+
+        var body = await OrderDownstreamCreateOneBodyBuilder.BuildAsync(
+            order,
+            senders.FirstOrDefault(),
+            recipients.FirstOrDefault(),
+            declarations,
+            weights,
+            CountryRepository);
+
+        return (order, body);
+    }
+}
diff --git a/src/Oms.Application/Shared/OrderDownstreamCreateOneBodyBuilder.cs b/src/Oms.Application/Shared/OrderDownstreamCreateOneBodyBuilder.cs
new file mode 100644
index 0000000..1a29df2
--- /dev/null
+++ b/src/Oms.Application/Shared/OrderDownstreamCreateOneBodyBuilder.cs
@@ -0,0 +1,183 @@
+using Oms.Application.Dtos.Orders;
+using Oms.Domain.OrdersAggregate.Consts;
+using Oms.Domain.OrdersAggregate.Entities;
+using Oms.Domain.SystemAggregate.Repositories;
+
+namespace Oms.Application.Shared;
+
+/// <summary>
+/// 从订单表及关联子表组装 createOrderOne 标准请求体。
+/// </summary>
+public static class OrderDownstreamCreateOneBodyBuilder
+{
+    public static async Task<OrderParcelCreateInput> BuildAsync(
+        Order order,
+        OrderSender? sender,
+        OrderRecipient? recipient,
+        IReadOnlyList<OrderDeclaration> declarations,
+        IReadOnlyList<OrderWeight> weights,
+        ICountryRepository countryRepository,
+        CancellationToken cancellationToken = default)
+    {
+        var countryIds = new List<int>();
+        if (sender?.Country is > 0)
+        {
+            countryIds.Add(sender.Country.Value);
+        }
+
+        if (recipient?.Country is > 0)
+        {
+            countryIds.Add(recipient.Country.Value);
+        }
+
+        var countries = countryIds.Count == 0
+            ? []
+            : await countryRepository.GetListAsync(x => countryIds.Contains(x.Id));
+        var idToCode = countries.ToDictionary(x => x.Id, x => x.CountryCode);
+
+        var forecast = weights.FirstOrDefault(x =>
+                            string.Equals(x.Type, OrderWeightTypeCodes.Forecast, StringComparison.OrdinalIgnoreCase))
+                       ?? weights.FirstOrDefault();
+
+        return new OrderParcelCreateInput
+        {
+            RequestId = order.RequestId,
+            ClientNo = order.ClientNo,
+            TrackNo = order.TrackNo,
+            ServiceNo = order.ServiceNo,
+            ProductCode = order.ProductCode,
+            ChannelCode = order.ChannelCode,
+            CargoAttribute = order.CargoAttribute,
+            ExportCustomsMode = order.ExportCustomsMode,
+            WarehouseId = order.WarehouseId,
+            CustomerId = order.CustomerId,
+            Length = forecast?.Length,
+            Width = forecast?.Width,
+            Height = forecast?.Height,
+            ActualWeight = forecast?.ActualWeight,
+            VolumetricWeight = forecast?.VolumetricWeight,
+            ChargeableWeight = forecast?.ChargeableWeight,
+            Cubic = forecast?.Cubic,
+            IsCod = order.IsCod > 0,
+            CodCurrency = order.CodCurrency,
+            CodSum = order.CodSum,
+            DeclaredCurrency = order.DeclaredCurrency,
+            DeclaredSum = order.DeclaredSum,
+            Quantity = order.Quantity,
+            Receiver = MapReceiver(recipient, order.ConsigneeCountry, idToCode),
+            Sender = MapSender(sender, idToCode),
+            Items = declarations.Select(MapItem).ToList()
+        };
+    }
+
+    private static string? ResolveCountryIso(int? countryId, string? orderConsigneeCountry, IReadOnlyDictionary<int, string> idToCode)
+    {
+        if (!string.IsNullOrWhiteSpace(orderConsigneeCountry))
+        {
+            var trimmed = orderConsigneeCountry.Trim();
+            if (trimmed.Length == 2)
+            {
+                return trimmed.ToUpperInvariant();
+            }
+        }
+
+        if (countryId is > 0 && idToCode.TryGetValue(countryId.Value, out var code) && !string.IsNullOrWhiteSpace(code))
+        {
+            return code.Trim().ToUpperInvariant();
+        }
+
+        return null;
+    }
+
+    private static OrderParcelReceiverInput? MapReceiver(
+        OrderRecipient? recipient,
+        string? orderConsigneeCountry,
+        IReadOnlyDictionary<int, string> idToCode)
+    {
+        if (recipient == null)
+        {
+            return string.IsNullOrWhiteSpace(orderConsigneeCountry)
+                ? null
+                : new OrderParcelReceiverInput { CountryCode = orderConsigneeCountry.Trim().ToUpperInvariant() };
+        }
+
+        return new OrderParcelReceiverInput
+        {
+            RecipientsName = recipient.RecipientsName,
+            CompanyName = recipient.CompanyName,
+            Phone = recipient.Phone,
+            TelePhone = recipient.TelePhone,
+            Email = recipient.Email,
+            CountryCode = ResolveCountryIso(recipient.Country, orderConsigneeCountry, idToCode),
+            Province = recipient.Province,
+            City = recipient.City,
+            StreetNumber = recipient.StreetNumber,
+            StreetAddress1 = recipient.StreetAddress1,
+            StreetAddress2 = recipient.StreetAddress2,
+            StreetAddress3 = recipient.StreetAddress3,
+            ZipCode = recipient.ZipCode,
+            TaxType = recipient.TaxType,
+            TaxNo = recipient.TaxNo,
+            CertificateType = recipient.CertificateType,
+            CertificateNo = recipient.CertificateNo,
+            Eori = recipient.Eori,
+            Vat = recipient.Vat
+        };
+    }
+
+    private static OrderParcelSenderInput? MapSender(OrderSender? sender, IReadOnlyDictionary<int, string> idToCode)
+    {
+        if (sender == null)
+        {
+            return null;
+        }
+
+        return new OrderParcelSenderInput
+        {
+            SenderName = sender.SenderName,
+            Company = sender.Company,
+            Phone = sender.Phone,
+            TelePhone = sender.TelePhone,
+            Email = sender.Email,
+            CountryCode = ResolveCountryIso(sender.Country, null, idToCode),
+            Province = sender.Province,
+            City = sender.City,
+            ZipCode = sender.ZipCode,
+            StreetAddress1 = sender.StreetAddress1,
+            StreetAddress2 = sender.StreetAddress2,
+            StreetAddress3 = sender.StreetAddress3,
+            HouseNumber = sender.HouseNumber
+        };
+    }
+
+    private static OrderParcelItemInput MapItem(OrderDeclaration x) =>
+        new()
+        {
+            CnName = x.CnName,
+            EnName = x.EnName,
+            Sku = x.Sku,
+            Unit = x.Unit,
+            UnitPrice = x.UnitPrice,
+            UnitWeight = x.UnitWeight,
+            Quantity = x.Quantity,
+            Amount = x.Amount,
+            Weight = x.Weight,
+            ImHsCode = x.ImHsCode,
+            ExHsCode = x.ExHsCode ?? x.HsCode,
+            SkuImage = x.SkuImage,
+            Category = x.Category,
+            Brand = x.Brand,
+            BrandType = x.BrandType,
+            IsMagnetic = x.IsMagnetic > 0,
+            Distribution = x.Distribution,
+            Material = x.Material,
+            EnMaterial = x.EnMaterial,
+            Purpose = x.Purpose,
+            Attributes = x.Attributes,
+            OriginCountry = x.OriginCountry,
+            SalesLink = x.SalesLink,
+            Model = x.Model,
+            Specifications = x.Specifications,
+            Comment = x.Comment
+        };
+}
diff --git a/src/Oms.Application/Shared/OrderDownstreamCredentialResolver.cs b/src/Oms.Application/Shared/OrderDownstreamCredentialResolver.cs
new file mode 100644
index 0000000..f665651
--- /dev/null
+++ b/src/Oms.Application/Shared/OrderDownstreamCredentialResolver.cs
@@ -0,0 +1,98 @@
+using Fake.DependencyInjection;
+using Fake.Application;
+using Oms.Application.Integrations.Itfx;
+using Oms.Domain.ChannelsAggregate.Repositories;
+using Oms.Domain.OrdersAggregate.Entities;
+
+namespace Oms.Application.Shared;
+
+public sealed class OrderDownstreamCredentials
+{
+    public string SupplierCode { get; set; } = string.Empty;
+
+    public string AppKey { get; set; } = string.Empty;
+
+    public string AppSecret { get; set; } = string.Empty;
+}
+
+/// <summary>
+/// 解析订单下游 itfx 凭证（优先订单绑定字段，回退渠道分组）。
+/// </summary>
+public class OrderDownstreamCredentialResolver(
+    IServiceChannelRepository serviceChannelRepository,
+    IServiceChannelGroupRepository serviceChannelGroupRepository,
+    IItfxMetadataClient itfxMetadataClient) : IScopedDependency
+{
+    public async Task<OrderDownstreamCredentials> ResolveAsync(Order order, CancellationToken cancellationToken = default)
+    {
+        if (!string.IsNullOrWhiteSpace(order.DownstreamSupplierCode)
+            && !string.IsNullOrWhiteSpace(order.DownstreamAppKey)
+            && order.DownstreamAppId is > 0)
+        {
+            var app = await itfxMetadataClient.GetAppAsync(order.DownstreamAppId.Value, cancellationToken);
+            var appSecret = app?.AppSecret?.Trim();
+            if (string.IsNullOrWhiteSpace(appSecret))
+            {
+                throw new BusinessException("应用 AppSecret 未配置");
+            }
+
+            return new OrderDownstreamCredentials
+            {
+                SupplierCode = order.DownstreamSupplierCode.Trim(),
+                AppKey = order.DownstreamAppKey.Trim(),
+                AppSecret = appSecret
+            };
+        }
+
+        if (string.IsNullOrWhiteSpace(order.ChannelCode))
+        {
+            throw new BusinessException("订单未绑定服务渠道，无法预报下游");
+        }
+
+        var channel = await serviceChannelRepository.GetByChannelCodeAsync(order.ChannelCode.Trim(), cancellationToken);
+        if (channel == null)
+        {
+            throw new BusinessException($"未找到渠道「{order.ChannelCode}」");
+        }
+
+        if (!channel.GroupId.HasValue || channel.GroupId.Value <= 0)
+        {
+            throw new BusinessException($"渠道「{order.ChannelCode}」未关联服务商分组");
+        }
+
+        var group = await serviceChannelGroupRepository.FirstOrDefaultAsync(x => x.Id == channel.GroupId.Value, cancellationToken);
+        if (group == null)
+        {
+            throw new BusinessException("服务商分组不存在或已删除");
+        }
+
+        if (group.AppId <= 0)
+        {
+            throw new BusinessException($"服务商「{group.Name}」未配置应用");
+        }
+
+        if (group.ApiServiceProviderId <= 0)
+        {
+            throw new BusinessException($"服务商「{group.Name}」未配置 API 服务商");
+        }
+
+        var appInfo = await itfxMetadataClient.GetAppAsync(group.AppId, cancellationToken);
+        if (appInfo == null || string.IsNullOrWhiteSpace(appInfo.AppKey) || string.IsNullOrWhiteSpace(appInfo.AppSecret))
+        {
+            throw new BusinessException("应用 AppKey / AppSecret 未配置");
+        }
+
+        var sp = await itfxMetadataClient.GetServiceProviderAsync(group.ApiServiceProviderId, cancellationToken);
+        if (sp == null || string.IsNullOrWhiteSpace(sp.Code))
+        {
+            throw new BusinessException("API 服务商编码（supplierCode）未配置");
+        }
+
+        return new OrderDownstreamCredentials
+        {
+            SupplierCode = sp.Code.Trim(),
+            AppKey = appInfo.AppKey.Trim(),
+            AppSecret = appInfo.AppSecret.Trim()
+        };
+    }
+}