diff --git a/packages/shared/src/messages.ts b/packages/shared/src/messages.ts
index d657eed02..e7bbcf9bf 100644
--- a/packages/shared/src/messages.ts
+++ b/packages/shared/src/messages.ts
@@ -2,7 +2,7 @@ import { isArray, isObject } from './utils'
 
 const isNotObjectOrIsArray = (val: unknown) => !isObject(val) || isArray(val)
 // eslint-disable-next-line @typescript-eslint/no-explicit-any
-export function deepCopy(src: any, des: any): void {
+export function deepCopy(src: any, des: any, isRetainExistMessage?: boolean): void {
   // src and des should both be objects, and none of them can be a array
   if (isNotObjectOrIsArray(src) || isNotObjectOrIsArray(des)) {
     throw new Error('Invalid value')
@@ -17,7 +17,11 @@ export function deepCopy(src: any, des: any): void {
         // replace with src[key] when:
         // src[key] or des[key] is not an object, or
         // src[key] or des[key] is an array
-        des[key] = src[key]
+        // if you need to keep the existing message on the i18n instance when merging, 
+        // you do not need to perform the following replacement
+        if (!isRetainExistMessage || typeof des[key] === 'undefined') {
+          des[key] = src[key]
+        }
       } else {
         // src[key] and des[key] are both objects, merge them
         stack.push({ src: src[key], des: des[key] })
diff --git a/packages/vue-i18n-core/src/composer.ts b/packages/vue-i18n-core/src/composer.ts
index b5bc124e3..4c003090d 100644
--- a/packages/vue-i18n-core/src/composer.ts
+++ b/packages/vue-i18n-core/src/composer.ts
@@ -2458,7 +2458,8 @@ export function createComposer(options: any = {}): any {
   // mergeLocaleMessage
   function mergeLocaleMessage(
     locale: Locale,
-    message: LocaleMessageDictionary<Message>
+    message: LocaleMessageDictionary<Message>,
+    isRetainExistMessage: boolean = false
   ): void {
     _messages.value[locale] = _messages.value[locale] || {}
     const _message = { [locale]: message }
@@ -2470,7 +2471,7 @@ export function createComposer(options: any = {}): any {
       }
     }
     message = _message[locale]
-    deepCopy(message, _messages.value[locale])
+    deepCopy(message, _messages.value[locale], isRetainExistMessage)
     _context.messages = _messages.value as typeof _context.messages
   }