Survivability is an important guarantee of the combat system under network combat environment. To appraise a system properly, it's necessary to establish a valid method for evaluating the survivability. According to the characteristics of network war system, this paper studies the primary factors which have important influence on the survivability of such a system, and then abstracts these metrics to three layers, including the goal layer, the intermediate layer and the index layer. Furthermore, with the AHP (Analytic Hierarchy Process ) and gray theory, this paper proposes a way to evaluate this kind of system's survivability. As a result, some suggestions are given out to improve the system performance. In the end, we verify the validity of the method through an example.