A power supply system includes a power supply assembly, an auxiliary power circuit and a control unit. The power supply assembly converts input power into a first DC power when the input power outputted from an input power source is normal. The auxiliary power circuit includes at least one energy storage unit for providing a second DC power and power converter electrically connected between the energy storage unit and the load for converting the second DC power into an individual auxiliary power. The control unit drives the auxiliary power circuit to provide an overall auxiliary power to the load when the input power is normal and a transient power required by the load is greater than a upper limit rated value of an output power outputted from the power supply assembly, so as to compensate a difference value between the transient power and the upper limit rated value.