Aimed at the modeling issues in design and quick processing of extruding die for special-shaped products, with the help of Conformal Mapping theory, Conformal Mapping function is determined by the given method of numerical trigonometric interpolation. Three-dimensional forming problems are transformed into two-dimensional problems, and mathematical model of die cavity surface is established based on different kinds of vertical curve, as well as the mathematical model of plastic flow in extruding deformation of special-shaped products gets completed. By upper bound method, both vertical curves of die cavity and its parameters are optimized. Combining the optimized model with the latest NC technology, NC Program of die cavity and its CAM can be realized. Taking the similar extrusion of square-shaped products with arc radius as instance, both metal plastic similar extrusion and die cavity optimization are carried out.