after update trigger sends data to log table and sends tag_id .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;