Using the Star cd with UG software, the 3 dimensional external airflow around a car body is numerically simulated.The results are analyzed and discussed. The results show that the air flow adjacent to the lower surface of the car separates immediately at the front of the bottom, and the resulting disturbance makes the boundary layer increase speedy; and that the flow adjacent to the upper surface of the car separates at the very front of the rear window, which makes large part of the tail of the car immersed in the separated flow, and that due to pressure difference and wrapping of main flow, some complicated Vortexes produce in the wake region.