A power system fault diagnosis method based on Petri net theory and expert system is presented,the method based on outgoing line diagnosis model and fault equipment detection,fault equipment and their protection relevance are elaborated indetail,the topological adaptability of the model is increased through the nanlysis of direcivity,regional and relevance of the protection,therefore,this method overcomes the shortcoming that expert system is difficult to be described by mathematical method,increases speculative ability of Petri net on this problem and uses Petri net to set up mathematical model to boost fault diagnosis speed.The diagnosis results show that this method can greatly reduce response time and can better fit for the change of electrical power net configurations than previous method.