A power supply system includes a transformer for converting an external AC power into an external DC power, a battery for providing a battery power, a selection module for outputting the external DC power or the battery power, a first-stage DC conversion module for converting power provided by the selection module into a first DC power, a plurality of second-stage DC conversion module, each for converting an input power into a second DC power, and a switching module for switching to output the battery power or the first DC power of the first-stage DC conversion module as the input power of each of the plurality of second-stage DC conversion module according to the external DC power.