Abstract: It is so easy for using the conventional inner-outer submerged arc One-step welding process to make extralarge diameter, heavy wall thickness SAWH pipe to cause occurrence of inner seam defects like stress tearing, etc., and thus to lead low primary qualification rate of the seam. Such problem is be solved by means of optimizing the formation and welding process, modifying the welding equipment, and adopting the One-step Double-process method. At the same time the defects like slag inclusion and pore are also reduced. Thanks to the said method, the targets like production cost reduction, seam quality improvement and enhancement of productivity are all satisfied.