A power supply apparatus includes a switch, a converting module and a control module. When the switch operates in an ON state, an input voltage is outputted through the switch to serve as an output voltage. The converting module selectively converts the input voltage into the output voltage. When the power supply apparatus operates in a mode where the switch operates in an OFF state, where the converting module performs the conversion, and where the output voltage is stabilized at a target voltage value, and when a condition associated with the input voltage is met, the control module causes the output voltage to gradually change toward the input voltage.