diff --git a/at_client.py b/at_client.py
index 40f967a..5bb003d 100644
--- a/at_client.py
+++ b/at_client.py
@@ -76,10 +76,11 @@ class ATClient:
         """
         expect_b = expect.encode() if isinstance(expect, str) else expect
         with self._cmd_lock:
-            # 初始化等待
-            self._waiting = True
-            self._expect = expect_b
-            self._resp = b""
+            with self._q_lock:
+                # 初始化等待
+                self._waiting = True
+                self._expect = expect_b
+                self._resp = b""
 
             # 发送
             if cmd:
@@ -300,8 +301,8 @@ class ATClient:
                     if len(self._rx) > 512 * 1024:
                         self._rx = self._rx[-256 * 1024:]
                 else:
-                    if len(self._rx) > 16384:
-                        self._rx = self._rx[-4096:]
+                    if len(self._rx) > 32768:
+                        self._rx = self._rx[-16384:]
 
 
 
diff --git a/network.py b/network.py
index f18a763..e5beefb 100644
--- a/network.py
+++ b/network.py
@@ -721,7 +721,7 @@ class NetworkManager:
             if self._network_type == "wifi":
                 return self._check_wifi_connection()
             elif self._network_type == "4g":
-                return True  # 4G连接状态由AT命令维护
+                return self._check_4g_connection()
             return False
 
         # 自动选择网络
@@ -738,6 +738,37 @@ class NetworkManager:
             return self._connect_tcp_via_4g()
         return False
 
+    def _check_4g_connection(self):
+        """检查4G TCP连接是否仍然有效（通过查询PDP地址验证网络附着状态）"""
+        try:
+            atc = hardware_manager.at_client
+            if atc is None:
+                return False
+            if not self._uart4g_lock.acquire(timeout=3000):
+                # 获取锁超时说明有其他操作在进行，视为连接仍有效
+                return True
+            try:
+                r = atc.send("AT+CGPADDR=1", "OK", 3000)
+                m = re.search(r'\+CGPADDR:\s*1,"([^"]+)"', r)
+                ip = m.group(1) if m else ""
+                if ip and ip != "0.0.0.0":
+                    return True
+                # 无IP或IP无效，尝试重新激活PDP
+                self.logger.warning("[4G-TCP] PDP地址无效，尝试重新激活")
+                atc.send("AT+MIPCALL=1,1", "OK", 15000)
+                r2 = atc.send("AT+CGPADDR=1", "OK", 3000)
+                m2 = re.search(r'\+CGPADDR:\s*1,"([^"]+)"', r2)
+                ip2 = m2.group(1) if m2 else ""
+                if ip2 and ip2 != "0.0.0.0":
+                    return True
+                self.logger.error("[4G-TCP] 重新激活PDP仍无有效IP，连接已断开")
+                return False
+            finally:
+                self._uart4g_lock.release()
+        except Exception as e:
+            self.logger.warning(f"[4G-TCP] 连接检查异常: {e}")
+            return True  # 异常时不误判断线
+
     def _wrap_wifi_tls(self, plain_sock, hostname):
         """
         在已建立的 TCP socket 上做 TLS（WiFi 走主机 ssl 库；4G 仍用模组 AT+SSL）。
@@ -1085,6 +1116,8 @@ class NetworkManager:
                     total += n
 
                 hardware_manager.uart4g.write(b"\x1A")
+                with hardware_manager.at_client._q_lock:
+                    hardware_manager.at_client._rx = b""
                 r = hardware_manager.at_client.send("", "OK", 8000)
                 if ("SEND OK" in r) or ("OK" in r) or ("+MIPSEND" in r):
                     return True