Abstract: Described in the paper is the method for designing the modified sinking die of the rectangular flat steel tube. The design involves major modifications of the die, i.e., turning the original flat long-edge face into a curved one, and getting connecting part between the inlet transitional zone and the sizing zone having an oval-shaped concave curved surface. These modifications can not only prevent sudden outward change of the die itself due to the negative effect by heat treatment, but also reduce the pressure and friction imposed upon the long-edge face of the rectangular flat tube during the sinking process so as to avoid defective concavities on the long-edge faces of the tube after sinking.