A fault diagnosing method based on the time-space relation of protective relays is proposed. A fault associated tree is defined to describe the logical cooperation between protection and breaker. The priority of protection is established according to the time-space relation and operation time of protective relays. Then the priority is mapped onto the dynamic reasoning model of Petri nets to realize precise diagnosis of fault elements in the grid. At the same time， evaluation of the protections operation and breakers action is made according to the priority factors of protection. Simulation results show that the fault diagnosing method can make rapid diagnosis in complex fault situations and give accurate evaluation results.