Withmandrel drawing process and the mechanisms of smoothing deformation have been analyzed. A method of smoothing deformation is brought forward that rectangular tube should be extruded uniformity out of die land with little amount of deformation. The dynamics model simplified and FE model of withmandrel drawing process are established based on FEM software, then simulations are performed. Three points including the center of long side, the center of short side, and the center of corner are positioned for further analysis. As for lineartype and steptype mandrel lands, the effective plastic strain and contact stress of three key points are evaluated. As for the plastic strain at the center of long side, a most difficult place to ensure surface quality, steptype mandrel land can improve 45.5%. As for the contact stress, steptype mandrel land can increase to 13MPa from 0. Drawing with steptype mandrel land achieves the stress status tensile stress in one orientation and compressive stress in other two orientations, which purposes of little amount of deformation to correct thickness tolerance and improve inner and outer surface quality of rectangular tube.