The utility model discloses an on-line monitoring device for the capacitance of a parallel capacitor. The on-line monitoring device comprises current sensor modules, wireless terminal modules, a ZigBee transmit-receive module and a host computer. Each current sensor module is sequentially connected with one wireless terminal module, the ZigBee transmit-receive module and the host computer. Each current sensor module transmits a signal to one wireless terminal module, and the wireless terminal module uploads data to the host computer through the ZigBee transmit-receive module. Each wireless terminal module comprises a main control unit, an A/D conversion unit and a wireless transmit-receive unit, and the main control unit is connected with the A/D conversion unit and the wireless transmit-receive unit. The on-line monitoring device for the capacitance of the parallel capacitor is highly reliable and can perform monitoring timely, so that the workload of inspection and maintenance is greatly reduced and the stability of a mains supply system is improved, thereby playing an importance role in the normal power supply of the whole electric system.