Large SiC mirror is a new kind of primary mirror. There is no success support example for study. In order to support SiC larger mirror, the merits and drawbacks of different active supports methods were analyzed. Finally the hydraulic system parallel connection with force actuator ways was chosen. The free harmonic oscillation model scaling method was used to study the active correction force of some 4 m SiC mirror. It was found that the smallest correction force is 0.1 N. To design a force actuator with such a small force resolution, the method of how to design high precision actuator was studied. Main factors that affect the precision of force actuator were discussed. The force actuator was designed with a step motor/harmonic drive combination to drive lead screw/floating nut to produce small displacement. A complex spring system was used to transform the stroke of the screw into a force. The force actuator was produced and tested. It was found that force output range is -400 N-400 N, the displacement resolution is 0.96 μm and the force resolution is 0.05 N. These test results suggested that this force actuator can satisfied the need.