The invention discloses an assembly method of a hybrid power device, which adopts an integrated mounting system. The assembly method comprises the following steps: S1, mounting a range extender support device on an airframe of a hybrid power aircraft, and mounting the range extender on the range extender support device; S2, mounting that motor mounting plate on the range extend support device, andmounting the driving motor on the motor mounting plate; S3, mounting the first controller mounting plate on the airframe of the hybrid aircraft, and mounting the first controller on the first controller mounting plate. A method for assemble a hybrid power plant By adopting the integrated mounting system, the range extender, the drive motor and the controller of the hybrid electric power device can be integrated and installed, the assembly efficiency of the hybrid electric device and the compactness of the whole structure can be improved, the arrangement of the hybrid electric device on the hybrid electric aircraft is convenient, the dismounting and the mounting are convenient, and the maintenance and the operation are convenient.