Credibility is the key performance index and the lifeline for system simulation. Verification, validation and accreditation (VV&A) of all system simulation life can insure the credibility, and credibility evaluation is a very necessary approach to obtain the credibility. Combat simulation is a special simulation for its many characteristics of multi-level, more elements, overall emergence, uncertainty, morbid and multi-granularity model, and so on. Credibility is vital to combat simulation. The evaluation method is one of the aspects of credibility evaluation. In this paper, the methods of credibility evaluation are researched. The common evaluation methods are analyzed and compared. Aiming at the characteristics of combat simulation system, the subjective comprehensive evaluation method based on subject matter expert (SME) is put forward, which is applied to a combat simulation. The practice shows that the subjective comprehensive evaluation method is very effective. The results are sufficiently credible to support the application.