A power generation device and method using organic working fluid for circulation without pump and valve are invented, the principle is able to be called flash jet natural cycle. The flash injection tube, the magnetic sphere motion tube and the storage tank are connected to form the main loop. The finned tube and the storage tank are connected to form the cooling loop. The working fluid is heated in the flash injection tube by low-temperature heat source, and is cooled in the finned tube by air or water. The gravity and flash force jointly drive the flash jet natural cycle. The lower magnetic ball acts as a one-way valve for supplying liquid to the flash injection tube. The upper magnetic ball acts as a valve and a rotor driven by the flash force and gravity to move linearly and produce electricity.