diff --git a/src/JT808.Protocol/Internal/DefaultMsgSNDistributedImpl.cs b/src/JT808.Protocol/Internal/DefaultMsgSNDistributedImpl.cs
index fd91dc1..f978d77 100644
--- a/src/JT808.Protocol/Internal/DefaultMsgSNDistributedImpl.cs
+++ b/src/JT808.Protocol/Internal/DefaultMsgSNDistributedImpl.cs
@@ -1,21 +1,18 @@
-using System;
-using System.Collections.Concurrent;
-using System.Collections.Generic;
-using System.Threading;
+using System.Collections.Concurrent;
 using JT808.Protocol.Interfaces;
 
 namespace JT808.Protocol.Internal
 {
     internal class DefaultMsgSNDistributedImpl : IJT808MsgSNDistributed
     {
-        ConcurrentDictionary<string, ushort> counterDict;
+        readonly ConcurrentDictionary<string, ushort> counterDict;
         public DefaultMsgSNDistributedImpl()
         {
             counterDict = new ConcurrentDictionary<string, ushort>(StringComparer.OrdinalIgnoreCase);
         }
         public ushort Increment(string terminalPhoneNo)
         {
-            if (counterDict.TryGetValue(terminalPhoneNo, out ushort value)) 
+            if (counterDict.TryGetValue(terminalPhoneNo, out ushort value))
             {
                 ushort newValue = ++value;
                 counterDict.TryUpdate(terminalPhoneNo, newValue, value);
@@ -23,8 +20,8 @@ namespace JT808.Protocol.Internal
             }
             else
             {
-                counterDict.TryAdd(terminalPhoneNo, 1);
-                return 1;
+                counterDict.TryAdd(terminalPhoneNo, 0);
+                return 0;
             }
         }
     }
diff --git a/src/JT808.Protocol/JT808Header.cs b/src/JT808.Protocol/JT808Header.cs
index 354c310..aa9cb93 100644
--- a/src/JT808.Protocol/JT808Header.cs
+++ b/src/JT808.Protocol/JT808Header.cs
@@ -26,14 +26,14 @@ namespace JT808.Protocol
         /// 根据安装后终端自身的手机号转换。手机号不足 12 位，则在前补充数字，大陆手机号补充数字 0，港澳台则根据其区号进行位数补充
         /// (2019版本)手机号不足 20 位，则在前补充数字 0
         /// </summary>
-        public string TerminalPhoneNo { get;  set; }
+        public string TerminalPhoneNo { get; set; }
         /// <summary>
         /// 消息流水号
         /// 发送计数器
         /// 占用两个字节，为发送信息的序列号，用于接收方检测是否有信息的丢失，上级平台和下级平台接自己发送数据包的个数计数，互不影响。
         /// 程序开始运行时等于零，发送第一帧数据时开始计数，到最大数后自动归零
         /// </summary>
-        public ushort MsgNum { get;  set; }
+        public ushort MsgNum { get { return ManualMsgNum ?? 0; } set { ManualMsgNum = value; } }
         /// <summary>
         /// 手动消息流水号（only test）
         /// 发送计数器
@@ -63,7 +63,7 @@ namespace JT808.Protocol
             // 2.消息体属性
             jT808Header.MessageBodyProperty = new JT808HeaderMessageBodyProperty(reader.ReadUInt16());
             if (jT808Header.MessageBodyProperty.VersionFlag)
-            {            
+            {
                 // 2019 版本
                 // 3.协议版本号
                 jT808Header.ProtocolVersion = reader.ReadByte();
diff --git a/src/JT808.Protocol/JT808Package.cs b/src/JT808.Protocol/JT808Package.cs
index 8e00c65..b84b5eb 100644
--- a/src/JT808.Protocol/JT808Package.cs
+++ b/src/JT808.Protocol/JT808Package.cs
@@ -193,8 +193,7 @@ namespace JT808.Protocol
             else
             {
                 //  2.4.消息流水号
-                if (value.Header.MsgNum <= 0)
-                    value.Header.MsgNum = config.MsgSNDistributed.Increment(value.Header.TerminalPhoneNo);
+                value.Header.MsgNum = config.MsgSNDistributed.Increment(value.Header.TerminalPhoneNo);
                 writer.WriteUInt16(value.Header.MsgNum);
             }
             //  2.5.判断是否分包