This paper introduces a hidden 35 kV MOV defect detected by infrared detector. It is found that the MOV leakage current measurement under continuous operating voltage is large. After replacing its visual inspection, the insulation resistance testing and leakage current measurement to the voltage U_1mA and 0.75U_1mA under 1mA DC reveals that the leakage current is abnormal. The MOV disintegration shows that the MOV is damp. According to the detection, it is recommend to strengthen the process control of MOV and deploy infrared temperature measurement when humidity is high.