The direct-drive ring permanent magnet torque motor is easily affected by parameters changes and the load torque disturbances, which reduces the servo performance of the system. In order to enhance the robustness of the servo system, the super twisting algorithm based on the second order sliding mode control (SMC) is proposed as the speed controller of the direct-drive servo system. The super twisting algorithm need not know the information of the sliding mode time derivative, which through the continuous control measure the sliding mode and its derivative approach zero in finite time. This method not only guarantees the robustness of the servo system and eliminates chatting, but also enhances the static precision of the servo system. The simulation results show that the servo system of the direct-drive NC rotary table has a very strong robustness by adopting the control method against parameters changes and the external disturbances.