A power supply system includes a power conversion circuit, a conversion control circuit, an auxiliary power module, an electricity supply module and a power supply control circuit. The power conversion circuit converts an input power into an output power used as an output of the power supply system. The conversion control circuit is arranged for controlling an operation of the power conversion circuit according to a control power. The auxiliary power module is arranged for selectively outputting a first electrical power to the conversion control circuit. The electricity supply module is arranged for providing a second electrical power. The power supply control circuit is coupled to the auxiliary power module and the electricity supply module, and arranged for referring to at least the second electrical power to determine whether to provide the first electrical power or the second electrical power as the control power for the conversion control circuit.