An automatic door operator includes a lead screw having a first end connected to an output end of an actuating unit and a second end extending away from the output end. A lead screw nut is attached to the lead screw. A slider is attached to the lead screw nut. The slider is provided with a rack portion extending in a direction parallel to an axial direction of the lead screw, and during a movement of the lead screw nut, a length of an overlap of the rack portion and the lead screw in the axial direction varies. The operator includes a housing having a guide hole configured to be slidably engaged with the slider and an output shaft is mounted in a mounting hole disposed on the housing. The output shaft has a gear portion positioned in the guide hole and engaged with the rack portion.