Abstract: In this paper, a comprehensive judgement method is proposed, in Which multiple factors influcing the dam safety are qualitatively、quantitavely and mixedly analysed on the fruitful experience of experts. Comprehensive inference judgement are applied to reveal essence of things. Thereby, the structural behaviour of a dam can be synthetically judged, and it’s possible development tendency can be forcasted. The discussion centres on fundemental prmciples of the method, which was applied to judge the stability of left abutment of a gravity arch dam. A hidden danger was found. Some measures were put foward to solve the problems.Good results have been a chieved.?