Manually adjusting the flow rate is difficult to ensure the accuracy and response speed when high temperature valve works at high temperature conditions, and there is also security risk. Therefore, the automatic control of high temperature valve opening becomes actual need of the project. In the design of the opening degree control system, opening degree was converted into displacement. A mathematical model of displacement control system was established after a detailed analysis of servo valve controlled hydraulic cylinder system and a closed-loop PID controller parameters of the valve opening were designed after simulating the closed loop characteristics of this servo system by using the simulation software Simulink. When the PID parameters are applied to the actual controlling, it is shown that the closed-loop servo control system has good response speed, stability and accuracy; the minimum controled valve opening degree reaches 2%. Its response is fast when the valve opening degree changes a lot and its response is slow when the valve opening degree changes a little. Although there is a deviation between experimental and simulation results, the results meet engineering requirements.