An AC power supply testing method upon booting a main board is provided. The testing method is applied to a main board, a microprocessor is in connection with the main board, and a relay is connected with the microprocessor, an AC power supply and a system power supply of the main board. The testing method includes steps of (1 ) configuring a system power on/off time of the microprocessor; (2 ) counting down the system power on/off time; (3 ) determining the current state of the relay; (4 ) shutting off the relay and resuming executing step (1) if the current state is ON; and (5 ) executing a power supply testing upon booting if the current state is OFF.