In the course of multistage forging for producing gear blank, the preforging design of gear blank directly affects metal flowing patterns of the finish forging, the situation of the forging die cavity, the quality of the products and die life. The network model between the size parameters of preforging and maximum forming force and maximum die stress of finish forging was established. Using the SGPLVM algorithm to reduce input variables to latent variables corresponding to the low dimensional space, through the relationship between the latent variables and output variables, the most superior shape and size parameters of preforging were determined by using GA algorithm. The results show that the preform is the optimal when both the maximum finisher forming force and the maximum finisher die stress are minimum.