The dewatering body is formed by a screw axis with fixing and moving plates overlapping, as the inner diameter of screw axis is larger than the moving plate, the moving plate do the circular motion with the screw axis to prevent clogging. The space between the fixing and moving plates becomes smaller and smaller along the direction of mud discharge. After gravity concentration, the sludge is transported to the dehydration parts, it dewaters under the inner pressure of outlet back plate.