In order to enhance the practicability of hydraulic mechanical continuously variable transmission（ HMCVT） in the field of vehicle,a new method based on improved simulated annealing algorithm and genetic algorithm for optimal design of transmission parameters of automobile HMCVT is presented in this paper. In this paper,a light cargo vehicle is taken as an example. The new method takes into account three aspects: maximum transmission efficiency, matching vehicle design requirements with transmission characteristics and lightweight. The two-stage method is used to optimize the transmission parameters of a three-stage HMCVT. Specifically,10 first stage parameters of planetary characteristic and gear pair transmission ratio are optimized by using genetic algorithm to optimize transmission efficiency and transmission characteristics; 33 secondary parameters of gear tooth number,modulus,helical angle and tooth width with the aim of lightening are optimized by using improved simulated annealing algorithm. The results of optimization design show that the new design method proposed in this paper has better optimization effect and can improve the design efficiency of HMCVT to a great extent; average transmission efficiency increased by 5%; the transmission performance can match vehicle driving requirements,and the design error is 0. 62%; the light weight effect of transmission is obvious.