We analyze and compare several typical existing methods of security situation assessment. Aiming at subjective randomness which exists in present security situation assessment methods, and the problems of largescale, complex structure and frequent information exchange of the information system, based on the principle of analytic hierarchy process (AHP), we built an index system for the information system. Aiming at the coherence problem that exists in present fuzzy analytical hierarchy process (FAHP), we propose a new consistency modification algorithm and apply it to assessing security situation. Meanwhile, we employ the fuzzy comprehensive evaluation (FCE) method to calculate situation value, and establish a new security situation assessment model, called AHPIFAHPFCE. We describe the constitutional principles and methods, as well as the main tasks of every step. Experimental results show that the model is more effective and accurate than the existing models for security situation assessment of the information system.