Based on the study of the law for material distribution in combined extrusion process of the screw plug, a new formula of material distribution for roundcup/hexagonal-rod combined extrusion was educed, and the dividing flow section pattern was presented to be a paraboloid of revolution.  And then, a forming numerical simulation for screw plug with Deform was performed. A forming stress diagram and a pressure-stroke curve were obtained. The forming process of screw plug was actually divided into two phases: the combined extrusion and the backward extrusion. Finally, a trial-production was performed. The two actual values of forward extrusion exit and backward extrusion exit were obtained. These actual values are coincided with the theoretical value.