According to the operation logic of the relay protective unit and using the concept of Markov state space， this paper proposes a model for calculating the probability of correct failure removal of the relay protection system. By considering the effect of major protection and backup protection， the model can evaluate the reliability of a protection system with different configurations， truly reflecting the probability of its correct failure removal. The validity of the model is shown with an example. The results are of value for the power system’s reliability analysis and risk assessment.