An uninterruptible power supply and method for controlling same are disclosed. The controlling method includes the steps of bypassing the first AC power to the output terminal via the bypass loop and the switch and converting a second AC power having a voltage, phase and frequency substantially equal to that of the first AC power by an inverter when the first AC power is normal; and switching the second AC power to the output terminal via the switch when the phase or frequency of the first AC power is changed so as to generate a difference value between the first AC power and the second AC power and the difference value is larger than a predetermined difference value.