The actuator is driving unit used for active deformation of the main reflector system of FAST. It is a hydraulic cylinder integrated with low pressure, small pump displacement and gear pump control, which is innovatively developed for the special demand of active deformation of FAST and resist of rough outdoor environment. Its piston moves at low speed (<2mm/s), but with high dynamic positioning accuracy (error less than 0.02mm) and high position-holding accuracy under static full load (error less than 0.1mm/h).