Based on the fragility degree and the analytic hierarchy process（AHP），a dam risk assessment method using fuzzy mathematics as a tool is proposed in this paper.The risk classification of reservoirs is determined by the degree of membership.Risk analysis of four reservoirs in Dayu county shows that the risk of large and medium-sized reservoirs is low，while that of small reservoirs is high.The study shows that the fuzzy evaluation method based on vulnerability can objectively reflect the risk ranking of reservoirs and provide a new way for risk assessment of dams.