A power supply device and an operating method thereof are provided. The power supply device includes a main converter and an auxiliary converter. The main converter includes a power factor corrector (PFC), a first capacitor that connects in parallel with the PFC and a DC/DC converter that connects in parallel with the first capacitor. The auxiliary converter is connected in parallel to the main converter. When the power supply device operates in a normal mode, the main converter and the auxiliary converter together provide a first output to an output load. When the power supply device is in a standby mode, the DC/DC converter is turned off so that only the auxiliary converter provides a second output to the output load. Meanwhile, the PFC is in operation to maintain the voltage of the first capacitor in order to meet the demand of the output dynamic response of the main converter.