Car body gravity deformation analysis by finite element method, and roof cross members are easy to have big gravity deformation. By researching on the relationship between gravity deformation and part thickness, camber, rib shape, it can be concluded that gravity deformation will be reduced a lot by optimize part camber and rib shape. Roof cross member gravity deformation reduced 48% by optimization the structure at last. It can identify the quality risk in advance by finite element virtual analysis of part manufacturing deformation during body process simultaneous engineering phase, and it is helpful to instruct the part design and reduce manufacturing cost.