With sliding interface and moving mesh technique provided by STAR-CD software, the 3-D transient external flow field around two vehicles passing each other was simulated. The tendency of changing drag and side coefficient was provided. By analyzing the pressure distribution and velocity vector on some sections, the reasons for the tendency and transient aerodynamic characteristics were concluded. The data shows: in the case that two cars which have the same volume and shape pass each other, the tendency of changing drag coefficient is similar to negative sinusoid,and the side coefficient has three peak values;But in the case that a car and a container truck, whose volume and shape have huge differences, pass each other, the tendency is more complicated and intense.