Abstract: For further improvements on the efficiency and accuracy of fault diagnosis,a fault diagnosing method is proposed based on an and/or tree model.The and/or tree can describe a fault more exactly as the power grid topology and protection start-up messages are incorporated.The search tree is used to connect fault elements and circuit breakers which are obtained by the power grid topology.Then the and/or tree fault diagnosis model can be obtained when the or tree is substituted in.The or tree contains protection start-up messages,protection operation messages and circuit breaker trip off messages.The answer tree can judge the fault sections and optimum match algorithm can help find the missing alarm messages.Simulation results show that the proposed method can meet the requirements of practical applications.