An uninterruptible power supply includes an energy storage unit, an AC-to-DC converting circuit, a charging circuit, a selection circuit, a primary detecting circuit, a primary control circuit and a secondary control circuit. The primary detecting circuit is connected to an output loop of the uninterruptible power supply for detecting an operating status of the uninterruptible power supply and issuing a corresponding operating message. The primary control circuit is connected to the primary detecting circuit and the selection circuit for controlling operations of the uninterruptible power supply. The secondary control circuit is connected to the primary detecting circuit, the selection circuit and the primary control circuit for controlling operations of the uninterruptible power supply. The primary control circuit and the secondary control circuit are enabled or disabled according to the operating message.