1. A computer-implemented method for tagging a document, the method comprising using a processor coupled to a memory: acquiring a user-input tag; building a structural information repository and a tag document index repository by processing a predetermined document; acquiring structural information of the document; retrieving a content block list corresponding to the user-input tag, wherein the content block list is stored in the tag document index repository; comparing a content block in the structural information acquired with a content block in the content block list, thereby obtaining a similar block; and annotating the user-input tag at a position corresponding to the similar block in the document, thereby creating a tagged document.