Abstract: An on-line arithmetic was presented, coupling fluent with Matlab method, based on two-way coupling method of aerodynamics and system dynamics of vehicle to analysis the stability of a van in transient crosswind conditions. Compared with conventional analysis method, the new method achieves obvious results, when the van runs in crosswind condition of 9 m/s for 1 second, the final lateral displacement, force and yaw moment has a respective reduction of 3.3%、4.8% and 17.8%,the corresponding slip velocity, yaw velocity have a relative difference more than 6%. Moreover the gaps will enlarge with the time increasing. The analysis results indicate that, it should not be ignored that the aerodynamics has an effect on the state of vehicle moving; it is necessary to consider synthetically the effect of yaw and slip velocity, because the lateral force to vehicle is not always increase with yaw angle increasing; two-way coupling method has set up a close combination between aerodynamics and system dynamics of vehicle, being more suitable to actual circumstance.