A power supply system for a circuit device includes a power supply unit, a switch unit and a voltage detector. The power supply unit, coupled to the circuit device via a power line, is used for supplying basic power for the circuit device via the power line, wherein the power line is coupled to the circuit device via a plurality of nodes. The switch unit, near to a node among the plurality of nodes, is coupled to the circuit device via the power line. The voltage detector, coupled to the circuit device and the switch unit, is used for detecting a voltage of the node and controlling the switch unit to be closed to allow the circuit device to receive auxiliary power via the switch unit when detecting that the voltage of the node is lower than a first threshold value.