In order to study the influence of size and structure of non-smooth surface on vehicle aerodynamics, we took MIRA stepped-back model as the research object, and studied the aerodynamics of three different models by using CFD method and wind tunnel experiment. Compared with smooth surface, the principle of resistance reduction was investigated. Simulation results show that the resistance reduction effect of non-smooth units originally arranged at the trunk deck，the rear of the car and underbody, is best with the ratio of 5.90%. Non-smooth surface shows good ability to reduce model pressure drag through refining eddy of the rear of body. Meanwhile, air velocity and body frictional resistance were reduced by changing the air flow of the near wall.