Abstract: In view of the excessive strength and excessive mass of a special vehicle gearbox, it is necessary to further optimize the box. In the optimization process, the traditional method based on the empirical formula of speed and torque has provided a larger dynamic load factor with conservative calculation, reducing the subsequent optimization space. In this paper, a lightweight design method was proposed based on load spectrum of bench test to get dynamic load coefficient and stress constraint of the vehicle gearbox. Firstly, a stress characteristic distribution model of the box body was established to screen the location of the measuring points. Secondly, the box test was carried out and the load spectrum of dangerous parts was measured. Then the load spectrum was processed to obtain the dynamic load coefficient and the supporting force was corrected for the box body. Finally, topology optimization and size optimization were carried out to achieve the goal of lightweight.