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A novel porthole extrusion process combined with corner extrusion principle for AZ91 pipe was presented. This new extrusion process can not only extend the length of the welding chamber, but also increase the rigidity of the latch needle, thereby ensuring the dimensional accuracy of pipe. Meanwhile, this new extrusion method can increase the deform degree and improve the dynamic recrystallization of the pre-welded metal, which can improve the welding quality and pipe quality. Then, the metal flowing characteristic, distribution feature of effective strain and mean stress in the corner porthole chamber were revealed; the results show that the effective strain of the separated metal becomes larger after flowing through the corner of the welding chamber, which can promote the quality of the welding seam. And the mean stress in the welding chamber is larger than 240 MPa, which satisfies the welding pressure condition. The influence rules of extrusion speed, die angle and billet preheating temperature on the mean stress in the welding chamber were also discussed; the result shows that the higher extrusion speed and the higher preheating temperature of the billet can enhance the mean stress in the welding chamber, which can improve the welding quality. The larger die angle can lead to higher mean pressure.