Defects of protective device seriously affect its operation reliability. Device defect distribution is an important basis for evaluating device reliability. Due to the long running time of the device, the defect occurrence rate is low, it is unable to obtain complete defect data, and the defect data has a random truncation feature. The maximum likelihood estimation method is used, and the distributed parameter estimation of protection devices is realized based on Weibull distribution model with two parameters. Based on the assumption that the power module, CPU module, AC sampling module and LCD module of the protection device have independent distribution characteristics, it builds joint distribution model, which realizes the parameter estimation of each module and the overall defect probability distribution model. The proposed method can effectively guide the device defect distribution analysis and the device maintenance. This work is supported by Science and Technology Project of State Grid Corporation of China:Relay Protection Device and Running Assistant Decision-making Technique (No. JBB17201600239).