The invention discloses a relay protection defect distribution evaluation method and system based on random truncated data, includes taking the defect-free running time of each device module as a random variable according to the defect occurrence of device modules of different modules of the same type of relay protection device during observation period, and taking the random variable of the device module without defect as a random variable, wherein the random variable of the device module without defect has the characteristics of random censoring data; Determining a probability density function of a two-parameter Will distribution of each type of device module according to the observed random truncated data; The two parameters are a shape parameter beta and a scale parameter eta; The likelihood function of defect distribution of each device module is established, and the maximum likelihood estimation of shape parameter and scale parameter of each device module is calculated accordingto the maximum likelihood function method and the observed random truncated data. According to the probability density function of the defect distribution of each type of device module, the joint distribution of the overall defects of the device is established, and the cumulative distribution of the overall defects of the relay protection device is evaluated.