A robust cascade control system based on active disturbance rejection controller (ADRC) is originally developed for accurate position control of a pneumatic servo system, which is usually characterized by nonlinearity, uncertainty, and disturbance. The proposed control system consists of inner and outer control loops. Particularly, a linear ADRC (LADRC) is utilized to adjust valve position for a linear spool valve dynamics in the inner loop. A nonlinear ADRC (NADRC) is designed to control the position of a nonlinear pneumatic actuator subsystem in the outer loop. LADRC and NADRC contain linear and nonlinear extended state observers (ESOs), respectively. The ESO can estimate both unknown nonlinear dynamics and external disturbances and thus make the ADRC robust against system uncertainties and disturbances. Simulation results successfully demonstrate the effectiveness and robustness of the proposed cascade control system. The stabilities of the inner and outer control loops were proved separately using a Lyapunov approach.