In order to obtain fine optical imaging quality, large astronomical telescopes commonly utilize active support system to correct surface deflection of the primary mirror. A device of pneumatic force actuator for telescope primary mirror support is designed and tested. Based on the principle of pneumatic systems, a type of pneumatic force actuator with a rolling diaphragm element is designed for primary mirror active support. In the actuator, a force decoupler with ball bearing is adopted to eliminate lateral force and flexural torque. In addition, a proportional-integral-differential (PID) control algorithm with dead zone is designed for the actuator. When the telescope′s elevation angle changes with the speed of 2°/s, the support force output can vary from 0 to 1000 N and the error is only ±0.4% of the full range. The experimental results show that the pneumatic force actuator designed is with good performance in static and dynamic force output.