A method for performing file search using a control computer receives an initial query expression from a client computer, divides the initial query expression into one or more keywords, determines related terms corresponding to each keyword, merges the keywords and the related terms to obtain an updated query expression, and generates search results according to the updated query expression. The method further calculates relevance scores among a plurality of documents in the search results and the updated query expression, ranks the documents in the search results according to the relevance scores, and displays the documents in the search results with the ranked order.