In this work, to reduce the drag of a van body truck, an aerodynamic device is fixing on the aft-body with different angle of declination and shape of the wing. The effects of reducing drag are studied by the methods of Numerical Simulation. And then the mechanism is discussed. The paper selects four rear add-on devices with different obliquity by 5deg, 10deg, 15deg and 20deg. And the shape of the wing is defined as straight line and curve, respectively. Seen from the results of numerical simulation, the rear add-on device can reduce the drag effectively. When the declination angle is 15deg, the effect is the best and the drag of the truck model is reduced by 8.9%. Furthermore, when the add-on plate is changed from contouring with straight line into Witozinsky curve in the same declination angle, the drag can reduce about another 3%. With the optimum declination angle and wing shape, the drag of the truck model is reduced 11.98%.