A power supply device and a single photo-coupler control circuit thereof are provided. The power supply device includes a single photo-coupler control circuit, a main converter and an auxiliary converter. The main converter connects to an output load. The auxiliary converter connects in parallel with the main converter. The single photo-coupler control circuit, also connected to the output load, monitors the status of the power supply device and controls the main converter and the auxiliary converter accordingly. The single photo-coupler control circuit only has one photo-coupler. The main converter and the auxiliary converter together provide a first output to the output load when the power supply device works in a normal mode. The main converter is turned off so that only the auxiliary converter provides a second output to the output load when the power supply device works in a standby mode.