The gearboxes, which make up the core component of the metro train bogie, should be lightweight and highly reliable in order to guarantee the security and comfort of train operation. The transient dynamic analysis is performed on a certain type of domestic metro train gearbox as the research object to obtain the load borne by the gearbox body during operation. Then, the strength of the box is analysed. Finally, a lightweight design is completed using the topology optimization design method under the condition that the strength performance of the box is not reduced. The findings reveal that the weight of the optimized box is 145.8 kg, 12.27% reduction; the maximum deformation is 0.05981 mm, 14.46% reduction; and the maximum stress is 66.34 MPa, 7.41% reduction.