Abstract: The metal oxide arrester is widely used in power system. It's the main protection device avoiding over voltage. But after long running, it's insulation maybe decrease because of heating. This paper introduces an classical case that using live detection technologies such as Infrared Thermography and resistive current detection etc to find the damp fault inside the arrester. Through the analysis of the routine tests datas, judge the damp fault inside the arrester firstly. Then dismantle the arrester to prove the analysis right and analyze the reason of the fault. At last give some advices for routine operation and maintenance.