As a core protection device for power transmission system, metal oxide surge arresters must be monitored online. On the basis of studying the on-line monitoring methods commonly used in metal oxide surge arresters, this paper proposes a novel resistive current extraction method of metal oxide surge arresters based on harmonic analysis. The method employs the quasi-synchronous DFT harmonic analysis linear correction algorithm to measure the resistive current of metal oxide surge arresters. The simulation experiment and on-site experiment show that this method is feasible, effective and has high application value.