Numerical simulation of unsteady flow fields around overtaking and overtaken cars by two different overtaking spaces between cars were conducted with computational fluid dynamics(CFD) method. Spalart-Allmaras turbulence model, QUICK discrete scheme, SIMPLEC pressure and velocity coupling arithmetic and unstructured tetrahedral mesh and dynamic mesh technology were adopted. Both car models were simplified fastback reduced scale car models. The side forces, rolling moments and yawing moments of the cars during the overtaking process were calculated. 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 variation tendencies of the side force, rolling moment and yawing moment of the overtaken car by two different spaces are the same. But the variation tendencies of these forces are different from each other; The variation tendencies of the side force, rolling moment and yawing moment of the overtaking cars by two spaces are different apparently.