1. A stop word identification method, wherein the method is executed by an information retrieval system running on a computing device and comprises: receiving a first query; acquiring a session identifier (ID) corresponding to the first query; acquiring, according to the session ID, a second query that belongs to a same session as the first query; acquiring a change-based feature of each word in the first query relative to the second query; and identifying a stop word in the first query according to the change-based feature of said each word in the first query relative to the second query.