diff --git a/src/main/java/run/halo/app/service/impl/PostCategoryServiceImpl.java b/src/main/java/run/halo/app/service/impl/PostCategoryServiceImpl.java
index 8985d9ae28..1731bcf558 100644
--- a/src/main/java/run/halo/app/service/impl/PostCategoryServiceImpl.java
+++ b/src/main/java/run/halo/app/service/impl/PostCategoryServiceImpl.java
@@ -4,7 +4,6 @@
 import java.util.Collection;
 import java.util.Collections;
 import java.util.HashMap;
-import java.util.HashSet;
 import java.util.LinkedHashSet;
 import java.util.LinkedList;
 import java.util.List;
@@ -124,9 +123,11 @@ public List<Post> listPostBy(Integer categoryId) {
         Assert.notNull(categoryId, "Category id must not be null");
 
         // Find all post ids
-        Set<Integer> postIds = new HashSet<>();
-        categoryService.listAllByParentId(categoryId).forEach(
-            x -> postIds.addAll(postCategoryRepository.findAllPostIdsByCategoryId(x.getId())));
+        Set<Integer> postIds = categoryService.listAllByParentId(categoryId)
+            .stream()
+            .map(item -> postCategoryRepository.findAllPostIdsByCategoryId(item.getId()))
+            .flatMap(Set::stream)
+            .collect(Collectors.toSet());
 
         return postRepository.findAllById(postIds);
     }
@@ -137,9 +138,11 @@ public List<Post> listPostBy(Integer categoryId, PostStatus status) {
         Assert.notNull(status, "Post status must not be null");
 
         // Find all post ids
-        Set<Integer> postIds = new HashSet<>();
-        categoryService.listAllByParentId(categoryId).forEach(x -> postIds.addAll(
-            postCategoryRepository.findAllPostIdsByCategoryId(x.getId(), status)));
+        Set<Integer> postIds = categoryService.listAllByParentId(categoryId)
+            .stream()
+            .map(item -> postCategoryRepository.findAllPostIdsByCategoryId(item.getId(), status))
+            .flatMap(Set::stream)
+            .collect(Collectors.toSet());
 
         return postRepository.findAllById(postIds);
     }
@@ -150,9 +153,11 @@ public List<Post> listPostBy(Integer categoryId, Set<PostStatus> status) {
         Assert.notNull(status, "Post status must not be null");
 
         // Find all post ids
-        Set<Integer> postIds = new HashSet<>();
-        categoryService.listAllByParentId(categoryId).forEach(x -> postIds.addAll(
-            postCategoryRepository.findAllPostIdsByCategoryId(x.getId(), status)));
+        Set<Integer> postIds = categoryService.listAllByParentId(categoryId)
+            .stream()
+            .map(item -> postCategoryRepository.findAllPostIdsByCategoryId(item.getId(), status))
+            .flatMap(Set::stream)
+            .collect(Collectors.toSet());
 
         return postRepository.findAllById(postIds);
     }
@@ -168,9 +173,11 @@ public List<Post> listPostBy(String slug, Set<PostStatus> status) {
             throw new NotFoundException("查询不到该分类的信息").setErrorData(slug);
         }
 
-        Set<Integer> postIds = new HashSet<>();
-        categoryService.listAllByParentId(category.getId()).forEach(x -> postIds.addAll(
-            postCategoryRepository.findAllPostIdsByCategoryId(x.getId(), status)));
+        Set<Integer> postIds = categoryService.listAllByParentId(category.getId())
+            .stream()
+            .map(item -> postCategoryRepository.findAllPostIdsByCategoryId(item.getId(), status))
+            .flatMap(Set::stream)
+            .collect(Collectors.toSet());
 
         return postRepository.findAllById(postIds);
     }
@@ -186,9 +193,11 @@ public List<Post> listPostBy(String slug, PostStatus status) {
             throw new NotFoundException("查询不到该分类的信息").setErrorData(slug);
         }
 
-        Set<Integer> postIds = new HashSet<>();
-        categoryService.listAllByParentId(category.getId()).forEach(x -> postIds.addAll(
-            postCategoryRepository.findAllPostIdsByCategoryId(x.getId(), status)));
+        Set<Integer> postIds = categoryService.listAllByParentId(category.getId())
+            .stream()
+            .map(item -> postCategoryRepository.findAllPostIdsByCategoryId(item.getId(), status))
+            .flatMap(Set::stream)
+            .collect(Collectors.toSet());
 
         return postRepository.findAllById(postIds);
     }