In order to prolong the endurance mileage of pure battery vehicle, a kind of auxiliary hybrid electric vehicle is introduced. A study on the system design, dynamic component selecting and parameter matching of auxiliary hybrid electric vehicle is carried out. Then the working methods of traction control unit and auxiliary power unit are introduced in detail. The results of experiment of emissions and simulation of endurance mileage show that auxiliary hybrid electric vehicle has low emissions,and compared with the original pure battery vehicle it can extend endurance mileage considerably.