Abstract: The paper introduced a design scheme of online monitoring device of insulator leakage current of medium voltage switchgear, gave whole design scheme of the device, introduced designs of amplifying circuit, filtering circuit, signal processing circuit and electromagnetic compatibility in details, and tested performances of the device by use of equivalent resistance-capacitance network under low voltage. When sine wave of 4 V is as input of the device, the output voltage is 2.2~3.8 V and the output current is 10~14 mA, which both meet design requirement and match pretty well in waveform. Besides，the output wave of the device is as the same as the measured wave under condition of certain phase shift, which shows that the device can realize function of online monitoring of insulator leakage current of medium voltage switchgear.