A power supply includes a casing mounted in a computer housing, a power module disposed in the casing, and an electrical socket and a switch module embedded in an exposed side wall of the casing. The switch module includes an operating unit mounted movably in a mounting seat, and operable so as to move among a first position adjacent to the side wall, where two terminal units coupled respectively to the electrical socket and the power module disconnect electrically from each other such that the switch module is in an OFF mode, a second position distal from the side wall, and a third position between the first and second positions, where a conducting member contacts electrically the terminal units such that the switch module is in an ON mode. When the switch module is switched from the ON mode to the OFF mode, the operating unit is moved from the third position to the first position via the second position.