The failure rate of digital protection device which is simulated by Weibull distribution and fitted by the L-M algorithm is constructed with the viewpoint of relay protection system in smart substation reliability. According to the character of imperfect maintenance, with the impacts of different maintenance’s types and the failure rate’s prediction of age reduction factor and hazard rate increase factor, the maintenance failure rate of digital protection device’s model can be extrapolated. With this model, the optimal strategy of non-regular maintenance intervals which is useful to running the device economic and reliable can be worked out. Analysis of example shows the effectiveness of this method.