diff --git a/src/JT808.Protocol.Test/Internal/JT808MsgSNDistributedTest.cs b/src/JT808.Protocol.Test/Internal/JT808MsgSNDistributedTest.cs
index 7f6b216..d658d25 100644
--- a/src/JT808.Protocol.Test/Internal/JT808MsgSNDistributedTest.cs
+++ b/src/JT808.Protocol.Test/Internal/JT808MsgSNDistributedTest.cs
@@ -1,8 +1,8 @@
-using JT808.Protocol.Interfaces;
-using JT808.Protocol.Internal;
-using System;
+using System;
 using System.Collections.Generic;
 using System.Text;
+using JT808.Protocol.Interfaces;
+using JT808.Protocol.Internal;
 using Xunit;
 
 namespace JT808.Protocol.Test.Internal
@@ -13,10 +13,11 @@ namespace JT808.Protocol.Test.Internal
         public void Test1()
         {
             IJT808MsgSNDistributed JT808MsgSNDistributed = new DefaultMsgSNDistributedImpl();
-            var a=JT808MsgSNDistributed.Increment("1234");
-            Assert.Equal(0, a);
-            var a1 = JT808MsgSNDistributed.Increment("1234");
-            Assert.Equal(1, a1);
+            for (int i = 0; i < 10; i++)
+            {
+                var a = JT808MsgSNDistributed.Increment("1234");
+                Assert.Equal(i, a);
+            }
         }
     }
 }
diff --git a/src/JT808.Protocol/Internal/DefaultMsgSNDistributedImpl.cs b/src/JT808.Protocol/Internal/DefaultMsgSNDistributedImpl.cs
index f978d77..d4e607c 100644
--- a/src/JT808.Protocol/Internal/DefaultMsgSNDistributedImpl.cs
+++ b/src/JT808.Protocol/Internal/DefaultMsgSNDistributedImpl.cs
@@ -14,8 +14,8 @@ namespace JT808.Protocol.Internal
         {
             if (counterDict.TryGetValue(terminalPhoneNo, out ushort value))
             {
-                ushort newValue = ++value;
-                counterDict.TryUpdate(terminalPhoneNo, newValue, value);
+                ushort newValue = value;
+                counterDict.TryUpdate(terminalPhoneNo, ++newValue, value);
                 return newValue;
             }
             else