diff --git a/internal/application/repository/chunk.go b/internal/application/repository/chunk.go
index 1bfd16ca..1e9a8ab1 100644
--- a/internal/application/repository/chunk.go
+++ b/internal/application/repository/chunk.go
@@ -90,7 +90,7 @@ func (r *chunkRepository) ListPagedChunksByKnowledgeID(
 	baseFilter := func(db *gorm.DB) *gorm.DB {
 		db = db.Where("tenant_id = ? AND knowledge_id = ? AND chunk_type IN (?) AND status in (?)",
 			tenantID, knowledgeID, chunkType, []int{int(types.ChunkStatusIndexed), int(types.ChunkStatusDefault)})
-		if tagID == "__untagged__" {
+		if tagID == types.UntaggedTagID {
 			// Special value to filter entries without a tag
 			db = db.Where("tag_id = ''")
 		} else if tagID != "" {
@@ -601,6 +601,7 @@ func (r *chunkRepository) UpdateChunkFlagsBatch(
 
 // UpdateChunkFieldsByTagID updates fields for all chunks with the specified tag ID.
 // Returns the list of affected chunk IDs for syncing with retriever engines.
+// newTagID: if not nil, updates tag_id to this value (empty string means uncategorized)
 func (r *chunkRepository) UpdateChunkFieldsByTagID(
 	ctx context.Context,
 	tenantID uint64,
@@ -609,6 +610,7 @@ func (r *chunkRepository) UpdateChunkFieldsByTagID(
 	isEnabled *bool,
 	setFlags types.ChunkFlags,
 	clearFlags types.ChunkFlags,
+	newTagID *string,
 	excludeIDs []string,
 ) ([]string, error) {
 	// First, get the IDs of chunks that will be affected (for is_enabled sync)
@@ -619,7 +621,7 @@ func (r *chunkRepository) UpdateChunkFieldsByTagID(
 			Select("id").
 			Where("tenant_id = ? AND knowledge_base_id = ? AND chunk_type = ?",
 				tenantID, kbID, types.ChunkTypeFAQ)
-		if tagID == "" {
+		if tagID == "" || tagID == types.UntaggedTagID {
 			query = query.Where("(tag_id = '' OR tag_id IS NULL)")
 		} else {
 			query = query.Where("tag_id = ?", tagID)
@@ -648,11 +650,20 @@ func (r *chunkRepository) UpdateChunkFieldsByTagID(
 		updates["is_enabled"] = *isEnabled
 	}
 
+	// Handle newTagID update (__untagged__ is stored as empty string)
+	if newTagID != nil {
+		if *newTagID == types.UntaggedTagID {
+			updates["tag_id"] = ""
+		} else {
+			updates["tag_id"] = *newTagID
+		}
+	}
+
 	query := r.db.WithContext(ctx).Model(&types.Chunk{}).
 		Where("tenant_id = ? AND knowledge_base_id = ? AND chunk_type = ?",
 			tenantID, kbID, types.ChunkTypeFAQ)
 
-	if tagID == "" {
+	if tagID == "" || tagID == types.UntaggedTagID {
 		query = query.Where("(tag_id = '' OR tag_id IS NULL)")
 	} else {
 		query = query.Where("tag_id = ?", tagID)
diff --git a/internal/application/service/knowledge.go b/internal/application/service/knowledge.go
index 926d31a2..b0d744e8 100644
--- a/internal/application/service/knowledge.go
+++ b/internal/application/service/knowledge.go
@@ -3570,7 +3570,7 @@ func (s *knowledgeService) UpdateFAQEntryFieldsBatch(ctx context.Context,
 			// Update all chunks with this tag
 			affectedIDs, err := s.chunkRepo.UpdateChunkFieldsByTagID(
 				ctx, tenantID, kb.ID, tagID,
-				update.IsEnabled, setFlags, clearFlags, req.ExcludeIDs,
+				update.IsEnabled, setFlags, clearFlags, update.TagID, req.ExcludeIDs,
 			)
 			if err != nil {
 				return err
diff --git a/internal/types/faq.go b/internal/types/faq.go
index 912653dc..2fa4cbea 100644
--- a/internal/types/faq.go
+++ b/internal/types/faq.go
@@ -228,6 +228,9 @@ type FAQSearchRequest struct {
 	MatchCount      int     `json:"match_count"`
 }
 
+// UntaggedTagID is the special tag ID representing uncategorized entries
+const UntaggedTagID = "__untagged__"
+
 // FAQEntryFieldsUpdate 单个FAQ条目的字段更新
 type FAQEntryFieldsUpdate struct {
 	IsEnabled     *bool   `json:"is_enabled,omitempty"`
@@ -243,7 +246,7 @@ type FAQEntryFieldsUpdate struct {
 type FAQEntryFieldsBatchUpdate struct {
 	// ByID 按条目ID更新，key为条目ID
 	ByID map[string]FAQEntryFieldsUpdate `json:"by_id,omitempty"`
-	// ByTag 按Tag批量更新，key为TagID（空字符串表示未分类）
+	// ByTag 按Tag批量更新，key为TagID（__untagged__表示未分类）
 	ByTag map[string]FAQEntryFieldsUpdate `json:"by_tag,omitempty"`
 	// ExcludeIDs 在ByTag操作中需要排除的ID列表
 	ExcludeIDs []string `json:"exclude_ids,omitempty"`
diff --git a/internal/types/interfaces/chunk.go b/internal/types/interfaces/chunk.go
index 1e6d09bd..72826053 100644
--- a/internal/types/interfaces/chunk.go
+++ b/internal/types/interfaces/chunk.go
@@ -64,8 +64,9 @@ type ChunkRepository interface {
 	// clearFlags: map of chunk ID to flags to clear (AND NOT operation)
 	UpdateChunkFlagsBatch(ctx context.Context, tenantID uint64, kbID string, setFlags map[string]types.ChunkFlags, clearFlags map[string]types.ChunkFlags) error
 	// UpdateChunkFieldsByTagID updates fields for all chunks with the specified tag ID.
-	// Supports updating is_enabled and flags fields.
-	UpdateChunkFieldsByTagID(ctx context.Context, tenantID uint64, kbID string, tagID string, isEnabled *bool, setFlags types.ChunkFlags, clearFlags types.ChunkFlags, excludeIDs []string) ([]string, error)
+	// Supports updating is_enabled, flags, and tag_id fields.
+	// newTagID: if not nil, updates tag_id to this value (empty string means uncategorized)
+	UpdateChunkFieldsByTagID(ctx context.Context, tenantID uint64, kbID string, tagID string, isEnabled *bool, setFlags types.ChunkFlags, clearFlags types.ChunkFlags, newTagID *string, excludeIDs []string) ([]string, error)
 	// FAQChunkDiff compares FAQ chunks between two knowledge bases and returns the differences.
 	// Returns: chunksToAdd (content_hash in src but not in dst), chunksToDelete (content_hash in dst but not in src)
 	FAQChunkDiff(ctx context.Context, srcTenantID uint64, srcKBID string, dstTenantID uint64, dstKBID string) (chunksToAdd []string, chunksToDelete []string, err error)