The typical structures of relay protection systems in smart substation for different sampling and tripping mode based on universal design are built. Both the complete reliability evaluation models and reliability analysis for relay protection system are also finished based on reliability block diagram. Probability sensitivity and component sensitivity are used to provide the element sensitivity and importance under the direct-sampling-direct-tripping mode protection system. The former is applied to find the weak link in order to improve the system design level, and the latter is used to search for the importance of each device to guide the operation & maintenance of the protection system. Research results can be used for the planning, operation & maintenance, and development for relay protection system in smart substation.