According to the requirement of a large telescope driven by a permanent magnet synchronous motor( PMSM) for pointing precision and following precision, a sliding mode position controller was designed based on a friction model with backstepping approach. Firstly , the system model was established based on LuGre friction model and external disturbance. Then, a subsystem which is the furthest away from the control input was designed by inversion method. In the design process, the sliding mode control law was induced into each step of backstepping approach to attenuate the influence of nonlinearity friction and external disturbance on the pointing precision and following precision of the large telescope. The effectiveness of the proposed method was verified by theoretical simulation and experimental test. The experimental results demonstrate that the system has a good dynamic performance and is insensitive to the uncertainty factors such as disturbance. The stable pointing error is 0.048 51″ when the position command is 4.6″, which decreases 21.4% than that of the traditional PI controller. The stable following error is 0.031 26″ when the ramp position command 5 (″)/s, which decreases 30.1% than that of the traditional PI controller. The proposed controller improves the pointing precision and following precision of large telescopes successfully.