Reducing vulnerability of social system is the foundation of social stability, if the social system becomes more and more vulnerable, then the social stability risk is greater, on the contrary, it is smaller, which put forward higher request for developing social risk detection platform. Risk detection is a kind of typical fuzzy pattern problem, a technology based on vulnerability evaluation is used for social stability risk. This paper analyzes the reasons of social vulnerability in the large hydraulic project construction area from three dimensions which are characteristic function of social risk exposure, social sensitivity and social adaptability capability, and builts the vulnerability evaluation model and risk detection index system, using entropy method to calculate the detection index weight. The paper finds that Wanzhou social vulnerability (social stability risk) present reduced year by year from 1999 to 2011, and the change caused by the enhancement of social adaptability capability, not by the fall of social system risk. The finding proves that social economic development, social security and public opinion promote the Social adaptability capability in Wanzhou.