The extrusion die is of great importance in the quality control of profile production. Yet in practice, the design of extrusion die is mainly dependent on the experience and intuition of die designers, which is difficult to guarantee product quality and productivity. In this paper, a numerical model was developed based on HyperXtrude with an attempt to investigate the effects of the shape of the welding chamber on metal flow and weld quality. The porthole dies with different steps of welding chamber were designed and applied to extrude an identical profile. Numerical results showed that with an increasing step of welding chamber, more uniform velocity and temperature distributions in the cross-section of the extrudate were observed. In addition, the weld quality was improved owing to increasing welding pressure, when adopting the multi-step welding chamber.