An exemplary actuator includes a base, a stationary frame mounted on the base, a moveable frame received in the stationary frame and supported on the base when the moveable frame is in a lowest position, and a driving unit received in the stationary frame and supported on the base. The moveable frame includes a frame body and a protrusion outwardly protruding from an outer wall of a corner of the frame body. The driving unit includes two parallel piezoelectric plates and two parallel resilient plates. The piezoelectric plates and the resilient plates are connected to each other end-to-end and cooperatively form a receiving hole therebetween. The protrusion is received in the receiving hole. The two resilient plates apply a preload to opposite side surfaces of the protrusion and retain the protrusion in position in the receiving hole.