An AC adapter including an AC-side plug module and a DC-side connector module coupled to each other is provided. A first voltage-reducing element and a relay are electrically coupled to an AC power supply. An AC-to-DC converter is electrically coupled to the relay. The DC-side connector module has a mechanical switch element electrically coupled to the relay and the first voltage-reducing element. When the DC-side connector module is connected to a device socket, the mechanical switch element is turned on and the relay is enabled, so that the electrical power of the AC power supply is transmitted to the AC-to-DC converter through the enabled relay. When the DC-side connector module is disconnected from the device socket, the mechanical switch element is turned off and the relay is disabled, so that the electrical power of the AC power supply is not transmitted to the AC-to-DC converter.