In order to improve the expanding performance of the clamping unit in vertical squeeze casting machine,the working principle and structure feature of double toggle clamping unit were analyzed,and the virtual prototype parametric model of the half clamping unit was established based on design points by using ADAMS software.The expanding force coefficient Mp（α=3°）was regarded as objective function for the optimization,and the optimum design variables were determined by analysis.At meeting the design constraints for vertical double-toggle clamping unit,the clamping unit was optimized by using Successive Quadratic Programming（SQP）.The optimized results show that the expanding force coefficient is increased by 5.02%,and the maximum clamping force is increased by 6.14%,realizing the goal of optimization and meeting the design requirements.