In order to achieve the effect of lightweight under the premise without weaken strength and stability of the structure,the main research object is based on the 2 AT gearbox which comes from the independent research and development of a car enterprises,establish its finite element model and its static analysis and modal analysis are carried out. Then,based on the analysis results,the stress,displacement,volume ratio and the first order natural frequency are the constraints. Finally,according to the optimization results,considering the manufacturing process and other requirements,the structure is improved in detail. The improved results show that the structural strength and the first order natural frequency are effectively improved,and the mass of the transmission case is reduced from 8. 238 kilogram to 7. 529 kilogram,which is reduced by 8. 6%,lightweight is realized.