The paper introduces some existing user identification algorithms, proposes IASR (IP, Agent, Session and Referrer) user identification algorithm to solve existing problems on user identification. The proposed algorithm overwrite URL in order to track users, efficiently and accurately identifies different users accessing the same proxy, and satisfactorily solves “Multi-User Problem” due to accessing Web via directly inputting URL in browser’s address bar. At last, the paper prospects future development of user identification algorithm.