Normal operation, the AC input through the rectifier filter circuit into a constant DC voltage, on the one hand to the DC battery float charge, on the other hand, the inverter is adjusted into AC pulse wave, and then filtered after shaping output stable power frequency AC. When the input AC power is off, the DC battery pack supplies power to the inverter to keep the AC power supply uninterrupted. When the inverter is faulty and the AC output is abnormal, the output electronic switch is off, the bypass electronic switch is closed, and the load is supplied by the bypass AC.