A backup power supply system supplies power from a first ac power source in normal condition. Once the first ac power source is abnormal or fails, the system automatically turns off the first ac power source and turns on a second ac power source with null transfer time. When the first ac power source is normal, a dc/ac power inverter is controlled to supply an approximately null current such that a mechanical switch is allowed to be closed and no circulating current is generated between the first ac power source and the dc/ac power inverter. Since the mechanical switch is continuously closed, and requires no switching operation, the backup power supply system can supply the backup power with a null transfer time.