An uninterruptible power supply includes an AC-to-DC converting circuit, an energy storage unit, a first path-switching circuit, a second path-switching circuit, an integrated charge/discharge circuit and an operating control unit. If the input voltage is abnormal, the input terminal and the output terminal of the integrated charge/discharge circuit are respectively connected to the energy storage unit and the power supply output terminal, so that electric energy stored in the energy storage unit is transmitted to the power supply output terminal through the integrated charge/discharge circuit. Whereas, if the input voltage is normal, the input terminal and the output terminal of the integrated charge/discharge circuit are respectively connected to the power supply output terminal and the energy storage unit, so that the energy storage unit is charged by the integrated charge/discharge circuit.