Abstract: At present, the accident analysis of power system mostly focuses on a single factor, and does not fully consider the new problems caused by the coupling of power system risk influencing factors. Based on the in-depth analysis of the actual operation environment of power system and the theory of accident causes, this paper first presents an analysis model of power system accident mechanism based on ''cognitive-constraint'', and systematically analyzes the origin of power system security risk from the aspects of human factors, equipment factors, management factors and environmental factors. Then, the fault tree analysis method based on Petri net is further constructed to quantitatively analyze the key factors affecting power system security. Finally, based on the power system accident cases, the model of the method is constructed, the structural importance of each factor is calculated, and the law of accident occurrence is preliminarily mastered. The comparison with the comprehensive analysis results of actual power system accidents shows that the model has good effectiveness and availability. The research results have certain reference significance for scientifically and reasonably avoiding power system accidents and building a safe power grid.