It is necessary to monitor the state of ZnO arresters to guarantee their safe operation. In order to select a best way to monitor the state of arresters, the advantages, disadvantages, and application of each monitoring method were analyzed. The results show that the harmonic analysis method is the most effective method. The properties of the harmonic analysis method in practical application could be improved by modifying the resistive leakage current and selecting an appropriate window function when conducting fast discrete Fourier transform on the collected signals. Through review of monitoring technologies in China and abroad, it can be found that in the future, the overall development trend of online monitoring methods of ZnO arresters is to develop fully automatic and multi-functional integrated online insulation monitoring systems.