To improve the anti-disturbance performance and following tracking accuracy for the servo system in a large telescope, a torque compensation method based on a disturbance observer was proposed. With the method, the revised acceleration/deceleration control method was adopt to guide the telescope turntable oscillating in a little angle. Through measuring the velocity and current of a motor, the rotation inertia of the telescope turntable was indentified. Then, an acceleration estimator was designed to estimate the low-acceleration based on encoder feedback data by using the double integration and PD control method. Finally, based on the inertia identification and acceleration estimation, a disturbance observer was designed to estimate the external torque according to the motor current and turntable acceleration. Furthermore, the estimated disturbance torque was used to compensate the current input to correct reference currents . Experiment results demonstrate that after the observer is added, the following error RMS is reduced from 0.012 7″ to 0.0073″ at the sloop position of servo system to be 0.36 (″)/s. Compared with that without the disturbance torque observer, the following tracking jitter is reduced and the tracking accuracy is improved.