In order to get under frame of certain type rocket launcher with more reasonable structure and lighter weight, choosing marching and launching conditions of under frame as typical working conditions, topology and size optimization were carried out on under frame by using structural optimization methods. The best material distribution of under frame was obtained through topology optimization. Based on this, design under frame of beam structures. The reasonable thicknesses of chassis beam structures were acquired through size optimization. The verification results show that the performance of the optimized under frame not only meets the stiffness and strength requirements but also its weight is reduced approximately 9% than the original under frame.