A set of force actuator was designed and tested for the active support system with a thin primary mirror to control the mirror surface error all the time. Conventional mechanisms for the force actuator with high accuracy and fine stability in engineering applications were studied. Then, based on the requirements of actuality and implementation in the thin primary mirror experiment system, a set of force actuator was designed and calculated in detail. A stepper motor integrated with a harmonic reducer was used to actuate the mechanism and precision screw to transmit, and an S-type Loadcell was taken as the force sensor to realize the feedback of force output. Finally, the mechanism was tested to validate the design feasibility through the opened loop and closed loop controls. Experimental results indicate that travel range of the force actuator is 0-10 mm, force range is -100-100 N and the accuracy is better than 0.05 N, which satisfies the demands of the force actuator for long range, high accuracy, fine output and stability. The actuator can be used in the support system of active optics, and is also a useful mechanism to other fine-tune structures.