A power supply system includes a first power module for outputting a first voltage and a second power module for outputting a second voltage. The first power module includes a first switch element, and a second switch element. The second power module includes a third switch element, and a fourth switch element. When the first power module is in a working state and the second power module is then plugged into the power supply system in a hot-swappable manner, only if a phase difference or a voltage difference between the first power source and the second power source is lower than a threshold value, the second switch element and the fourth switch element are controlled to be turned on. Subsequently, if the second voltage is higher than the first voltage, the third switch element is controlled to be turned on.