Aiming at the high reliability of the existing intelligent substation relay protection system, the failure sample data is too small to estimate the reliability parameters.Based on the analysis of the relay protection structure of smart substations, this paper proposes a method for assessing the reliability of relay protection of small sample data of smart substations by combining three-parameter Weibull distribution and gray estimation method, the three parameters of the distribution are estimated.The accuracy and rationality of this method are verified by comparing examples with other methods.Simulation results show that the algorithm proposed in this paper can quickly estimate reliability parameters based on real-time data changes, and at the same time ensure the accuracy of the estimation.This research provides reference and reference for the development of reliability assessment methods for relay protection systems in China.