In order to realize rational evaluation of the exit time of relay protection devices，an equipment failure rate model is developed first based on parameter estimation of the Marquardt method.Then，the equipment annual cost model related to the failure rate is built by analyzing the various factors of its life cycle cost.Meanwhile，an assessment strategy for equipment exit time based on the minimum annual cost is proposed.Finally，an example of a certain type of relay protection devices is given to seek the optimal equipment exit time.This method is useful for power production and planning departments to design rational equipment replacement policies.