The invention discloses an automatic feeding device in a full-automatic stacking container loader. The automatic feeding device comprises a rack, wherein a conveyer belt which is driven by a stepping motor and is used for conveying a flat distribution box is arranged in the rack; a rodless cylinder which is arranged along the operating direction of the distribution box is arranged on the rack on one side of the conveyer belt; a chute is formed in the cylinder body of the rodless cylinder along the axial direction; a slide block which can move along the chute is arranged on the chute; a mounting plate is fixed on the slide block; the mounting plate is hinged with a rear baffle through a connecting shaft; the rear baffle and the mounting plate are fixed with each other through a locking mechanism; the slide block moves back and forth along the chute under drive of a piston in the rodless cylinder; the rear baffle is synchronously driven to be always pressed against the rear side wall of the flat distribution box; multiple front stoppers are respectively arranged on the left and right sides of the front end of the rack above the output end of the conveyer belt; and all the front stoppers can be pressed against the front side wall of the flat distribution box. The device has the advantage of automatically feeding the distribution box.