A power supply system for memory modules includes a control unit and a voltage regulator. The control unit includes a basic input/output system (BIOS) and a control chip connected to the BIOS. The BIOS controls the control chip to output a control signal according to the number of the memory modules mounted in memory slots. The voltage regulator is connected to the control chip through first and second general purpose input/output (GPIO) buses. The voltage regulator receives the control signal from the control chip through the first and second GPIO buses and regulates power supply modes, to output different phase voltages to the memory modules mounted in the memory slots.