The research of autobody lightweight technology is an important mainstream in the autobody design. In this paper A FEM model for the unitary body of a home-made bus is established .Then optimization design variables are determined through stiffness sensitivity analysis and by using finite element analysis software, an optimization is conducted, based on which a lightweighting scheme is put forward. The experiment indicated that after appropriate optimization the mass of the whole car body decrease 19.35 Kg and the stiffness of the autobody maintained invariable.