Abstract: The technological process of shear-flash butt welding and the role of each stage were analysed in this paper.The key factor to welding quality is that the displacement of workpiece to be weld must be according with equation:s=kt².The mathematical model of hydraulic servo control system has been established.The basic principle of linear quadratic was described in detail.The simulation of hydraulic servo control system has been done by modeling and simulation software AMESim.LQR control algorithm and PID control algorithm were respectively used in simulation.The results showed that the tracking error of LQR controller is only one-sixth of traditional PID controller,which is much better meeting the requirements of the position servo system of shear-flash butt welding technological process.