A model of GA is given in this paper.As for the feature of GA, a selfadapting policy is drawn into mapped GA, and a mathematical model is provided.Meanwhile, a specific algorithm flowchart is given. Through the selfadapting scheme, the probability of cross and variation can be automatically adjusted  by the fit degree; the convergence speed and the quality of results are increased. The parameters of the system transfer function are achieved by this method, and an open loop model is developed. By comparing the simulation results of the open loop and the actual testing curves of the system, the possibility of using GA to gain the system parameters of hydraulic servo system is proved. Meanwhile, by analyzing the simulation results, the differences of the identification methods are found on the material testing machine's proportional and servo pressure control systems.