Aiming at the problems existing in traditional battery-powered electric vehicles, a supercapacitor plus bidirectional DC-DC converter is proposed as an auxiliary power source. This solution improves the energy efficiency of the entire system while making up for battery defects. Based on a brief introduction of the supercapacitor energy storage system, the core component of the system, the bidirectional DC-DC converter, is studied and designed. The mode switching strategy and control strategy of the bidirectional DC-DC converter are analyzed. The bidirectional converter performs a soft switching analysis. The corresponding waveform obtained from the prototype experiment verified the effectiveness of the scheme.