Abstract: In the reliability evaluation of relay protection, it is difficult to reflect the reliability level of the protection device in the current operating state by using the steady-state probability of the protection device state to perform state enumeration and consequence analysis. In this paper, a state transition model of relay protection device based on historical operation data was established, and a time-varying probability algorithm based on continuous-time Markov chain was proposed. The influence of different protection reliability parameters on the prediction model of relay protection behavior was analyzed and the parameter update method was proposed. Finally, according to the time-varying probability curve obtained from the model, an emergency protection device urgently invoking and implement strategy was proposed.The simulation results prove the objective law of the trend of the protection device behavior.