The invention discloses a power supply system applied to a power capacitor online monitoring system. The power supply system comprises a microwave energy transmitting device, a microwave energy receiving device, an energy storage device, a bidirectional charging and discharging circuit and a voltage-stabilizing circuit, wherein the microwave energy transmitting device and the microwave energy receiving device transmits energy through microwave signals, an output end of the microwave energy receiving device is connected with the energy storage device via the bidirectional charging and discharging circuit, and an output end of the bidirectional charging and discharging circuit is connected with a power supply end of the power capacitor online monitoring system via the voltage-stabilizing circuit. The power supply system is convenient in taking electricity, high in reliability and low in system power consumption, obtains milliwatt-to-watt level electric energy by means of a thin-film rectifying antenna array, stores the electric energy in a miniature graphene super-capacitor bank, outputs a stable voltage by means of the voltage-stabilizing circuit, and solves the signal power supply problem of the power capacitor online monitoring system.