A power supply device disposed on an aircraft to provide a power to the aircraft is provided. The aircraft has an average required power value. The power supply device includes a secondary battery, a transformer and a fuel cell. The transformer is coupled between the secondary battery and the aircraft. The fuel cell is coupled to the aircraft and is adapted to provide a first output current to the aircraft. The transformer has an output voltage set value. When the first output end voltage of the fuel cell is lower than the output voltage set value, the transformer provides a second output current of the secondary battery to the aircraft. The output voltage set value is in a voltage range with a fuel cell output power between the maximum power value of characteristic curve of the fuel cell and the average required power value of the aircraft.