Aiming at the issue of overweight in gantry-type wide-span operation platforms for agricultural machinery, research on structural lightweight design was conducted and a method to achieve structural weight reduction was proposed by addressing the high complexity of structural analysis models. The corresponding implementation process was also provided. Firstly, to tackle the complexity and detail density of the complete model, an equivalent elastic support modeling method was developed by using surrogate models, which significantly simplified the analysis model for noncritical regions while ensuring the accuracy of the simulation in the core areas and effectively reducing computational costs. Subsequently, sensitivity analysis was employed to identify key parameters affecting stress and mass in the design, addressing the issue of excessive design parameters resulting from component diversity. Finally, a lightweight design solution was obtained by using response surface optimization. Compared with the existing design, the optimized longitudinal beam structure reduced the mass by approximately 1 424 kg, achieving a 4.31% reduction in total vehicle mass, and the specific mass was decreased to 118.67 kg/kW, while still meeting strength and stiffness requirements. The lightweight design methodology established can provide a foundational approach and reference for the development of gantry-type wide-span operation platforms for agricultural machinery.