In order to meet the requirements of velocity control systems of large telescopes for the fast response,small overshoot,high steady-state accuracy and smooth running, a Variable Structure PID(VSPID) controller was proposed based on analyzing of classical PID control algorithm. By constructing the functions of control errors, such as proportional gain, integral gain, integral variable gain and differential gain, the VSPID was able to modify its structure and parameters according to the instantaneous error. Based on the transfer function model of certain large telescope,the function of VSPID was verified and the performance of classical PID was compared with that of the VSPID by simulation.A velocity control experiment was performed on this telescope,which shows that the telescope can reach the desired velocity with maximum acceleration and without overshoot. The steady-state error is less than 0.016 7 (°)/s at 20 (°)/s, and the maximum steady-state error is 0.7 (″)/s at 10 (″)/s. Simulation and experimental results prove that the velocity control system based on VSPID can meet the requirements of large telescopes.