To study the effect of the ground boundary on the numerical simulation of external flow field around the automobile,two schemes are made to numerically simulate the flow field around a mini-bus and the author analyzes the results.The study reveals that the different ground boundary greatly affects the aerodynamic lift and the front tire lift but has little effect on the aerodynamic drag and the rear tire lift.Precision can be improved on the basis of the moving ground in the numerical simulation of the external flow field around the automobile.