In order to better assess the reliability of protection system, this paper builds the Markov model of single protection system considering hidden failure and changes of operating condition. Then based on the model, the changes of hidden failure rate affected by power flow and the effect of hidden failure repair rate on the normal operation of protection devices can be acquired. We also put forward probabilistic model for the reliability assessment of UHV protection system, which is suitable for engineering applications. Probabilistic model is a simplification of Markov state-space model and provides a mathematical way for improving the protection system reliability assessment. The results verify the validity of the algorithm.