It is discovered in an analysis of a metal oxide arrester (MOA) fault that the partial discharge characteristic signal is not detected when testing the arrester with defective valve plate. To deeply analyze this phenomenon, the model of MOA is established by using COMSOL multiphysics^® simulation software. The electric field distribution law of MOA valve sheet surface is studied when there are defects at different positions and different types of defects. The surface electric field distributions of MOA valve sheet column with and without defects is compared, The sensitivity of partial discharge test to MOA valve defects is studied, and the simulation test is built to verify the results. The results of verification test are consistent with the simulation results. It is found that the partial discharge test is more sensitive to the defects of the MOA at both ends, and less sensitive to the defects of the MOA in the middle. The research results provide technical reference for the partial discharge field test of MOA.