Aiming at the lightweight design requirements of the large gear, which was a key component of the lumbar rehabilitation robot, this study was based on Altair Inspire software and adopts the method of topology optimisation to obtain a dendritic weight reduction model through OptiStruct analysis. After the verification of safety coefficient and shape variable, the weight reduction of the large gear was successfully achieved to be 4.115 kg, with a weight reduction ratio of 30.2%. The lightweight performance of five materials (Steel (AISI 1015), Steel (C45E), Titanium (Ti-17), ABS, and PEEK) was further compared. The results show that Titanium (Ti-17) has the lightest mass of 6.205 kg and the highest cost in the optimised design of metal materials. Plastic materials (ABS and PEEK) have significantly better lightweight performance than metal materials, especially PEEK, with an optimised mass of 1.597 kg, which is 1.25 times that of ABS, and a minimum factor of safety of 2.8, which is 2.5 times that of ABS. Considering the excellent overall performance of PEEK, it can be used as a preferred alternative material for the manufacturing of key components of lumbar rehabilitation robots. This study provides a structural optimisation and material selection scheme for the lightweight design of lumbar rehabilitation robots, and provides a reference for future related research and technology development.