The minimum maintenance model for relay is developed from the viewpoint of relay reliability and maintenance economy.Based on the proposed model，the optimal replacement strategy is proposed.Specifically，the Poisson distribution fitting and Cramer-von Mises goodness of fit test are used to determine the failure characteristics of protection devices，and the log-normal distribution is used to fit repair features of protection device.With the above failure and repair property，the minimum maintenance model is constructed.Furthermore，the unit life-cycle cost is applied in protection device reliability assessment to formulate the replacement strategy with the lowest unit life-cycle cost as the objective.Finally，the optimal replacement cycle for a field operating relay is obtained with the proposed strategy，with the double protection considered.The availability of the proposed strategy is verified by the simulation results.