diff --git a/Changelog.md b/Changelog.md
index 9d30c4ec4..24af63e66 100644
--- a/Changelog.md
+++ b/Changelog.md
@@ -32,6 +32,7 @@
 - 修复窗口较窄时，首页「当前节点」卡片中过长的节点名把延迟标签挤出卡片
 - 修复断开链式代理时清空已选节点，导致再次连接需要重新选择的问题
 - 修复「连接」页表格排序时表头换行、标题位置跳动的问题
+- 修复「TUN 设置」未手动保存时，界面显示的默认 MTU 1500 未生效的问题
 
 **🍎 macOS**
 
diff --git a/src-tauri/src/constants.rs b/src-tauri/src/constants.rs
index 3f85ced31..54ac3f309 100644
--- a/src-tauri/src/constants.rs
+++ b/src-tauri/src/constants.rs
@@ -70,6 +70,8 @@ pub mod files {
 pub mod tun {
     pub const DEFAULT_STACK: &str = "mips";
 
+    pub const DEFAULT_MTU: u16 = 1500;
+
     pub const DNS_HIJACK: &[&str] = &["any:53"];
 
     /// Saved dialog fields; `enable` belongs to the verge switch.
diff --git a/src-tauri/src/enhance/mod.rs b/src-tauri/src/enhance/mod.rs
index 08b790823..e84e2c20c 100644
--- a/src-tauri/src/enhance/mod.rs
+++ b/src-tauri/src/enhance/mod.rs
@@ -459,7 +459,7 @@ fn gui_tun_keys(clash_config: &Mapping) -> Vec<Value> {
         keys.extend(
             constants::tun::GUI_KEYS
                 .iter()
-                .filter(|key| tun.contains_key(**key))
+                .filter(|key| tun.get(**key).is_some_and(|value| !value.is_null()))
                 .map(|key| Value::from(*key)),
         );
     }
@@ -531,6 +531,16 @@ fn ensure_lan_bind_address(mut config: Mapping) -> Mapping {
     config
 }
 
+fn ensure_tun_default_mtu(mut config: Mapping) -> Mapping {
+    if let Some(Value::Mapping(tun)) = config.get_mut("tun")
+        && matches!(tun.get("mtu"), None | Some(Value::Null))
+    {
+        tun.insert(Value::from("mtu"), Value::from(constants::tun::DEFAULT_MTU));
+    }
+
+    config
+}
+
 /// Merges the app's clash config over the profile; switches are parameters to keep it pure.
 fn merge_default_config(
     mut config: Mapping,
@@ -548,7 +558,9 @@ fn merge_default_config(
             });
             let patch_tun = value.as_mapping().cloned().unwrap_or_else(Mapping::new);
             for (key, value) in patch_tun.into_iter() {
-                tun.insert(key, value);
+                if !value.is_null() {
+                    tun.insert(key, value);
+                }
             }
             config.insert("tun".into(), tun.into());
         } else {
@@ -878,6 +890,7 @@ pub async fn enhance(
 
     notify_discarded_keys(authoritative.overridden(&authoritative, &authoritative.current(&config)));
     let config = authoritative.enforce(config);
+    let config = ensure_tun_default_mtu(config);
     let config = ensure_lan_bind_address(config);
 
     let config = cleanup_proxy_groups(config);
diff --git a/src/components/setting/mods/tun-viewer.tsx b/src/components/setting/mods/tun-viewer.tsx
index 0ebeb8f73..69953c037 100644
--- a/src/components/setting/mods/tun-viewer.tsx
+++ b/src/components/setting/mods/tun-viewer.tsx
@@ -54,7 +54,7 @@ export function TunViewer({ ref }: { ref?: Ref<DialogRef> }) {
     autoDetectInterface: true,
     dnsHijack: ['any:53'],
     strictRoute: false,
-    mtu: 1500,
+    mtu: 1500 as number | undefined,
     dnsMode: '',
   })
 
@@ -374,14 +374,15 @@ export function TunViewer({ ref }: { ref?: Ref<DialogRef> }) {
             autoCapitalize="off"
             spellCheck="false"
             sx={{ width: 250 }}
-            value={values.mtu}
+            value={values.mtu ?? ''}
             placeholder="1500"
-            onChange={(e) =>
+            onChange={(e) => {
+              const next = Number.parseInt(e.target.value, 10)
               setValues((v) => ({
                 ...v,
-                mtu: parseInt(e.target.value),
+                mtu: Number.isNaN(next) ? undefined : next,
               }))
-            }
+            }}
           />
         </ListItem>