Pneumatic systems are widely used to drive a control valve in many kinds of process plant. In this paper, we propose a pneumatic system: this is composed of a cylinder type pneumatic actuator and a PWM (Pulse Width Modulation) positioner, because it is necessary to improve the performance for a control valve. The positioner has four on-off valves; they are controlled by a PWM mode. Examinations of the system are as follows. First, we show that it is possible to improve the characteristics of the control valve by adopting PWM method. Second, we indicate the main parameters that influence the performance especially the dynamic stiffness - of the system by linearizing basic equations and showing block diagram. Final, we show that the performance of the proposed system is better than traditional one.