Large injection-molded parts are prone to significant warpage deformation during the injection molding process due to their large size and complex rib structures. Traditional methods based on process parameter optimization and empirical structural design are often insufficient to meet the low deformation requirements of these parts. To effectively reduce such warpage deformation, a structural optimization design approach based on the extension innovation is proposed. This approach began by constructing an extension model of the injection-molded part, followed by extension analysis and extension transformation to generate multiple candidate optimization schemes. Then, injection molding simulations were employed to identify the optimal structural scheme. Simulation results showed that the warpage deformation on the bottom surface of the initial injection-molded building formwork was 3.49 mm, which was reduced to 2.82 mm after optimization using the extension innovation method. Final trial mold validation results demonstrated that the maximum relative error between the simulated value and measured average value of warpage deformation for the optimized injection-molded building formwork was only 5.6%. The extension innovation-based structural optimization design approach enables efficient identification of structural defects and rapid generation of optimized solutions, providing novel optimization insights and practical references for the low warpage design and manufacturing of large injection-molded parts.