A preparation method of small and ultra-thin power semiconductor device comprising the steps of: providing a chip mounting unit with a plurality of pads with a plate arranged on top surface of each pad; flipping and attaching a semiconductor chip on the chip mounting unit, where the electrodes at the front of the chip are electrically connected to the pads; forming a plastic packaging body covering the chip mounting units and the chip, where the top surface of the plate and the back surface of the chip are exposed out from top surface of the plastic packaging body and the bottom surfaces of the pads are exposed out of the bottom surface of the plastic packaging body; forming a plurality of top metal segments arranged on the top surface of the plastic packaging body and electrically connected to the top surface of each plate and the back surface of the chip.