An uninterruptible power supply includes plural power modules and a controlling module. The power modules are electrically connected with a power source, an energy storage unit and a load for converting an input voltage into an output voltage. The controlling module is electrically connected with the plural power modules and the energy storage unit for detecting a storage voltage of the energy storage unit and dynamically adjusting the output voltage of the plural power modules according to the storage voltage of the energy storage unit. If the controlling module is abnormal, the magnitude of the output voltage is maintained at a constant voltage value.