diff --git a/src/Oms.Application/Integrations/SmallBagTrack/SmallBagTrackUpsertRequestBuilder.cs b/src/Oms.Application/Integrations/SmallBagTrack/SmallBagTrackUpsertRequestBuilder.cs
index 19eef09..53a5d17 100644
--- a/src/Oms.Application/Integrations/SmallBagTrack/SmallBagTrackUpsertRequestBuilder.cs
+++ b/src/Oms.Application/Integrations/SmallBagTrack/SmallBagTrackUpsertRequestBuilder.cs
@@ -68,10 +68,18 @@ internal sealed class SmallBagTrackUpsertRequestBuilder(
     }
 
     /// <summary>
-    /// 仅取运单跟踪号；无值则空串，不回落运单号/客户单号等。
+    /// 轨迹侧 TrackNo 必填。有真实跟踪号用跟踪号；尚未下发时先用客户单号占位，后续有 TrackNo 再 Upsert 更新。
     /// </summary>
-    internal static string ResolveTrackNo(Order order) =>
-        order.TrackNo?.Trim() ?? string.Empty;
+    internal static string ResolveTrackNo(Order order)
+    {
+        var trackNo = order.TrackNo?.Trim() ?? string.Empty;
+        if (!string.IsNullOrWhiteSpace(trackNo))
+        {
+            return trackNo;
+        }
+
+        return order.ClientNo?.Trim() ?? string.Empty;
+    }
 
     internal static string? PickSupplierCodeFromProviders(
         IEnumerable<SalesProductAllocationProviderSnapshotDto> providers,
diff --git a/src/Oms.Tests/SmallBagTrackUpsertRequestBuilderTests.cs b/src/Oms.Tests/SmallBagTrackUpsertRequestBuilderTests.cs
index df70e79..47147c0 100644
--- a/src/Oms.Tests/SmallBagTrackUpsertRequestBuilderTests.cs
+++ b/src/Oms.Tests/SmallBagTrackUpsertRequestBuilderTests.cs
@@ -20,14 +20,28 @@ public class SmallBagTrackUpsertRequestBuilderTests
     }
 
     [Fact]
-    public void ResolveTrackNo_EmptyWhenTrackNoMissing()
+    public void ResolveTrackNo_FallsBackToClientNoWhenTrackNoMissing()
     {
         var order = new Order
         {
             ServiceNo = "IDE2607221859123456",
+            ClientNo = "CNO-001",
             TrackNo = null
         };
 
+        Assert.Equal("CNO-001", SmallBagTrackUpsertRequestBuilder.ResolveTrackNo(order));
+    }
+
+    [Fact]
+    public void ResolveTrackNo_EmptyWhenTrackNoAndClientNoMissing()
+    {
+        var order = new Order
+        {
+            ServiceNo = "IDE2607221859123456",
+            TrackNo = null,
+            ClientNo = null
+        };
+
         Assert.Equal(string.Empty, SmallBagTrackUpsertRequestBuilder.ResolveTrackNo(order));
     }