After the failure of the power grid, it is important to diagnose and remove the fault components quickly and accurately in order to ensure the normal operation of the power grid. Analyzing the power grid structure, protection configuration and operation rules of circuit breaker, the states of suspected components and the malfunction conditions of protective relays and circuit breakers are imported into the fault hypothesis. The objective function is established respectively for suspicious fault buses and fault lines, and the influence factors among different levels of protections are introduced for correction. An automatic modeling method based on branch element correlation matrix is proposed after deeply excavating into the topological relations among the grid components. Finally, modified plant growth simulation algorithm (MPGSA) is used in IEEE 14-bus system and the results of examples show that this method has a good application prospect since it can effectively improve the fault tolerance and accuracy of diagnosis.