As the characteristics of nonlinearity，parameter variability with time and strong interference existed in the hydraulic servo system of flash butt welding process, the traditional proportion integration differential (PID) control was difficult to meet the requirements on the control precision. A third-order nonlinear Active Disturbance Rejection Control (ADRC) was designed based on the main influence factors and process requirements of hydraulic servo system. The mathematical model of the hydraulic servo system was established. The simulations of the hydraulic system were done in unity by AMESim and Matlab software. Moreover the simulation results were compared with that of using PID controller. The conclusion shows that: the ADRC controller is better than conventional PID controller to meet the requirements of hydraulic position servo control system, and the hydraulic servo system obtains the performance of strong robustness and anti-interference ability.