In order to solve the problem of high aerodynamic drag for truck, a study of applying a passive drag reduction technology to the van was carried out. A simplified truck model〖CD2〗Ahmed model was used. And a two-equation model, SST k-ω model, was adopted as the turbulence model in the CFD simulation, then the effect of attack angle of the bottom tail plate of the Ahmed body on the flow field and aerodynamics drag was investigated, including the drag coefficient, surface pressure distribution and the tail vortex structures. The results show that the angle of the bottom plate of the van can reduce the strength of the tail vortex and thus recover the surface pressures of the tail, which in return decreases the drag. It is found that the drag coefficient reaches its minimum value when the attack angle is 10 degree and a reduction of drag about 6% is obtained.