An online arrester monitoring system is composed of one or more arrester sensing units and a main IED. The arrester sensing units are connected in series in a hand in hand mode and connected to the main IED. Each arrester sensing unit comprises a leakage current sensor, an action current sensor, a signal processing and communicating module, a display module and an internal power supply module. By means of the online arrester monitoring system, the total leakage current, the resistive current and the number of action times of the arrester can be comprehensively monitored in real time, the measurement accuracy is high, the site adaption is high, and the application requirements of an intelligent substation can be met.