Point to the manufacturing process of variable-section rectangular tube, a four-die radial extrusion process was designed in this article. The general deformation law was researched in two-dimensions, the influence of deformation parameters on the deformation results was explored, and the method by using the mold anti-deformation to eliminate the concave defects was proposed. For the tube parts with common thickness to diameter ratio, the corresponding optimal die arc degree is obtained through simulation and optimization. After that, the two-dimensional deformation law is applied to three dimensions, and the methods to eliminate the defect are carefully studied and successfully realized. Taking a variable-section rectangular tube as an example, the experiment was carried out. The experimental results were compared with the simulation results, and the accuracy of the simulation results and the feasibility of the process were verified.