A method of lightweight about car body was developed based on the analysis of sensitivity and crashworthiness. First, the thickness of components on car body was used as design variables. The modal and stiffness of car body were the constraint. The weight of car body was the objective of optimization. The sensitivity of the thickness about modal and stiffness was obtained. Based on this sensitivity, the components whose thickness was not sensitive to modal and stiffness of car body were selected as the design variables. Using these design variables, the optimization design was carried out with the minimize weight of car body as objective. The result of optimization made 14.8kg weight reduction of car body, and guaranteed the performance of stiffness and modal. The simulations of side crash including whole car and occupant restraint system were carried out to the car after lightweight. The performance of crashworthiness and occupant restraint system was validated through comparing the results of before and after lightweight. In order to make the lightweight car body satisfied the requirement of crashworthiness, the thickness of components was modified again according to the results of side crash. The results showed the feasibility of the lightweight method based on sensitivity and side crash simulation.