Abstract: Cracking is found at the foot area of the welding seam of the large-sized X80 SAWL pipe as expanded. Relevant physical tests and chemical analysis are made to the fracture,and the results reveal the main elements which responsible for the cracking,including rather high welding heat input that makes the toughness of the metal of the HAZ greatly decreased,concentration of the stress in the base metal near the welding seam of the pipe that occurs before it is expanded,and extrusion that takes place between the HAZ of the tube and the expanding die. Based on the elements as identified,the welding process is improved,and thus the crack incidence rate is decreased from 2.00% to 0.12%.