There are two typical design solutions in the car body design and layout: concave-convex under-body and smooth under-body. However, the effects of the under-body structure on the crosswind stability of the car is not clear, and clarifying the effects not only provides reference data for the design and layout of the bottom structure, but also is a key technical problem for the crosswind stability evaluation of car. Firstly, the numerical calculation model of typical bottom structure cars was established, and the influence laws of different side wind speeds on the aerodynamic force and aerodynamic moment of the cars were analyzed. Secondly, based on the coupling method of automobile aerodynamics and automobile system dynamics, the crosswind stability analysis and evaluation model of typical bottom structure cars was established, and the influence laws and mechanism of the under-body structure on the lateral motion, yawing motion and side-slip motion of the car were studied. The study shows that the concave-convex bottom structure causes an increase in aerodynamic lift force, pitch moment, drag, and side force, which not only intensifies the lateral motion and yaw motion of the cars under crosswind, but also increases the risk of cars skidding. The lower the tire-road friction coefficient, the greater the effect of the concave and convex bottom structure on the lateral motion and yaw motion, and the smaller the effect on the limit wind speed of lateral slipping.