The factors which lead to large-area power outages are more than ever, when the complexity of the expansion power system is increasing. Therefore, it is important to assess the vulnerability of the complicated power system. This paper first proposes an objective, practical and adaptive vulnerability assessment system of complicated power system by use of Analytic Hierarchy Process, which based on multi-factors analysis of large-area outages. This system can find out the vulnerability point of the power system，and take advantages of the government, who can over-all plan, correspond and organize the social resource to reduce the vulnerability of power system and prevent large-area power outages, and the power enterprises who can maintaining the stability of power system by technology. Finally, the paper gives an assessment example of provincial power system.