after update trigger sends data to log table and sends tag_id .I need tag Name?

I Have 4 tables:

Documents, document_tag, doc_document_tag and log.

• In document: doc_id, Doc_name are saved
• In Doc_tag: Tag_id,Tag_name are saved

• In doc_document_tag multiple tags are save against one or more
  documents.

  for every table logs are generated.

My question is that in my case in doc_document_tag only tag_id is saved and I need tag_name

Based on your description, the Doc_Tag table should have the Tag_name you need.

In the future, giving actual table definitions, some sample data and the actual expected result will help in getting a more accurate and complete answer.

This query example might get you started

  SELECT DT.tag_name
  FROM dbo.logs L
  INNER JOIN dbo.doc_document_tag DDT
  ON DDT.Tag_id = L.Tag_id
  INNER JOIN dbo.Doc_Tag DT
  ON DDT.Tag_Id = DT.tag_id;