A switch power supply includes an AC/DC converter, a DC/DC converter, a battery charge-discharge switching circuit, and a detecting and controlling circuit. The AC/DC converter receives an input AC voltage and converts the input AC voltage into a DC voltage. The DC/DC converter converts the DC voltage into several working voltages required for operating the electronic device. The battery charge-discharge switching circuit is selectively connected to the AC/DC converter or the DC/DC converter and connected to the battery. The detecting and controlling circuit detects the input AC voltage. If the input AC voltage is abnormal, the detecting and controlling circuit issues a control signal to the battery charge-discharge switching circuit. In response to the control signal, the battery issues a discharge voltage to the DC/DC converter and the discharge voltage is converted into the working voltages by the DC/DC converter.