Aiming to realize the lightweight design of the road wrecker’s loading platform, an efficacious method combining material substitution and size optimization is proposed in this paper. The finite element model is established at first. According to the wrecker’s working process, two typical working conditions are determined. Following the proposed stiffness and strength judgment bases, the FEA of the loading platform is carried out under typical working conditions. Finally, on the premise of meeting the basic performance of the materials, the part thickness is reduced by replacing Q235 steel with Q460 high-strength steel. The optimal size is obtained after size optimization based on OptiStruct software. The improved results show that the total mass of the new loading platform decreased by 30.8% compared with the previous structure. The promoted structural performance meets the requirements, which verifies the practicability of the lightweight design method.