Abstract: Coal mining enterprises have their own power supply network. Underground production mainly depends on electric energy, which can ensure normal operation of mine ventilation, gas drainage and discharge of coarse coal. Lightning strike of mine power grid will lead to frequent occurrence of blackout accident and cause a serious threat to underground workers. So it is significant to do a good job of lightning protection work in mining area. Zinc oxide arrester without gap operating usually operates under power frequency and has a current through the valve piece. These will cause aging of valve piece, increase of impedance leakage and temperature, which will cause accidents. So it is necessary to make an online monitoring on the running status of arrester to avoid hidden danger. In this paper, a new extracting method of resistive current is put forward and a mathematical derivation is adopted. And a simulation based on Simulink is used to verify the correctness of extracting principle.