In order to achieve the simulation of properties of battery–ultracapacitor (BU) hybrid system, the principle of BU in parallel is analyzed and the hybrid energy storage system (HESS) is redeveloped based on ADVISOR platform for vehicle. In the paper, a structure of HESS is proposed for hybrid electric vehicle (HEV) and a solution to develop the proposed sub-model is provided. Performance of simulation is performed by redeveloping HESS for paralleling HEV. The simulation result shows that redevelopment of HESS is available and the peak power of battery is improved obviously.