The present invention provides an uninterruptible power system (UPS), the UPS comprising: a first converter having a first input terminal for receiving a first power source and a first output terminal for outputting a first voltage, wherein the first power source is a DC power or AC power and the first voltage is a DC voltage; a third converter having a third input terminal for receiving the second power source and the third output terminal connected to the first output terminal, wherein the second power source is a DC power; and a second converter having a second input terminal connected to the first output terminal and the third output terminal, and the second output terminal for outputting a second voltage corresponding to the first voltage.