Abstract: For incorrect operation of existing identification methods for UPS switching, an identification method for UPS rapid switching was designed by use of DSP technology. The method uses software lock mode, and adopts AC voltage amplitude and phase signals colleted and processed by DSP to measure power fail and voltage drop. When the measured value exceeds the setting one, USP would be switched. The experimental result shows the method can determine AC power fail and voltage drop, and achieve UPS switching in about 1 ms.