Abstract: A flyback power supply device and its control method that can exactly switch off the switch component each time when the output voltage of the transformer is changed from positive to negative. The power supply device includes a transformer, a primary and secondary switch circuit. The primary and secondary switch circuits are connected with the primary and secondary sides of the transformer, respectively. The secondary switch circuit includes a control circuit and a switch component. The control circuit is used to keep the voltage across the switch component fixed and make the resistance of the switch component inversely proportional to the output current of the transformer. As the output current of the transformer changes as an inverse triangle, the resistance of the switch component will approach infinite when the output current is zero and the switch component will be switched off automatically to make the power supply device function well.