A power supply system includes a power supply, a switch control circuit, a voltage rectifying circuit, and a trigger switch connected to the switch control circuit. The switch control circuit is connected between an alternating current (AC) power source and the power supply. The voltage rectifying circuit is connected between the AC power source and the switch control circuit to rectify an AC voltage into a direct current (DC) voltage to power the switch control circuit. When the computer system is powered off, the power supply fails to output a system voltage, and the trigger switch fails to be triggered, the switch control circuit disconnects the power supply from the AC power source. When the computer system is powered off, the power supply fails to output a system voltage, and the trigger switch is triggered, the switch control circuit connects the power supply to the AC power source.