The fault rate of voltage sag sensitive equipment is important reference information for power grid side and user side to carry out voltage sag mitigation measure. This paper presents a nonparametric estimation method for fault rate assessment of sensitive equipment. In view of the independence of the magnitude, duration and point on wave of the voltage sag, a method for evaluating the probability density function of the characteristic quantity of voltage sag is proposed based on the non-parametric estimation method, so as to construct the joint probability density function of the position distribution of the infection point of the voltage tolerance curve in the uncertain area. The affected conditions of the uncertainty region are divided into three regions for analysis, and the fault rate evaluation model of the sensitive device based on the U-T joint probability density function is proposed. A PC voltage tolerance test platform is built, and the proposed method is verified based on the test data. A theoretical simulation model is built and connected to IEEE standard test system to verify the proposed method with simulation data. Based on the measured data and the simulated data, the correctness and practicability of the proposed method are proved by comparing with the existing method.