Abstract: In order to ameliorate the credibility of relay protection reliability assessment towards smart substation, based on study of traditional substation reliability, a multi-state-space diagram is established for smart substation relay protection system, consisting of three subsystems which are relay protection equipment, communication subsystem and personnel subsystem. Each subsystem is modeled with different reliability evaluation methods and its effect on the protection system is analyzed. Markov state space method with fuzzy algorithm is applied to solve the general availability of protection system under different configuration structures and network structures. A practical example is calculated to validate the effectiveness and rationality of the models and algorithms built in this paper, which provides reliable basis for smart substation protection system reliability evaluation.