The working principle of hydrostatic hybrid vehicles(HHV) is discussed. A number of factors affecting the efficiency of hydrostatic transmission hybrid system are discussed. The mathematical model is established by using braking energy recovery capacity, power, fuel economy and vehicle cost as objective function, system parameters are used as the independent variables, and the multi-objective optimization process is compiled in MATLAB. The process takes the genetic algorithm (GA) as the optimization method, obtains the best system parameters through computing, and verifies the accuracy of optimization results through simulation.