A composite power supply includes a plurality of power cluster lines, at least one USB port and at least one DC port formed on a surface of a casing, and the power cluster lines, the USB port and the DC port are electrically connected to a circuit board and the circuit board includes a plug-and-play circuit electrically connected to the USB port and provided for an electronic device to access electric power through the USB port, so as to waive the inconvenience of booting a computer before using the electric power, and the circuit board includes a DC-DC conversion circuit electrically connected to the DC port, and a knob or a multi-stage switch in the DC-DC conversion circuit is provided for adjusting the output voltage of the power ports to improve the convenience of the application significantly.