Metal oxide arrester（MOA）is an important overvoltage protection component for power equipment. Its operating state not only relates to its own safety, but also affects the protected equipment, which even leads to ground faults in the power grid. Infrared detection, as a means of live detecting of electrical equipment, can detect the arrester potential defects under operating voltage in time. It is important to check whether MOA can continue to run on the service voltage, and avoid the expansion of defects into electricity accidents. In this paper, the infrared detection of 500kV MOA was taken as an example to analyze the thermal characteristics of the insulation damp arrester. Through the disintegration analysis of MOA, the reasons for the dampening of the arrester and the principle of insulation degradation were analyzed. At the same time, the timeliness and accuracy of the infrared diagnosis to detect MOA insulation defects were proved.