Abstract: The unsteady flow fields around overtaking and overtaken cars in overtaking process at two different speeds were numerically simulated with moving mesh technology.The side forces,rolling moments and yawing moments of the cars during the overtaking process were calculated and analyzed.The influences of the flow field on the aerodynamic forces were analyzed according to the distribution of velocity and pressure fields.The results show that the maximum side force,rolling moment and yawing moment of the overtaken vehicle become bigger,the maximum side force and rolling moment of the overtaking vehicle will become bigger,the yawing moment will become smaller with the increase of the overtaking speed.