Abstract: The common six types of the defective undercuts of the SAWL pipe are described here in the paper，based on which the causes for the said defect are investigated.And accordingly，specific preventative actions are proposed.The analysis result reveals that the major cause for the defective undercut of the pipe is that it is the stress changing of the molten bath during welding operation that causes reduction of the molten metal flow area or change of the flow direction.Accordingly，the defective undercut can be lessened by means of such actions as adjusting the process parameters，bevel position，and welding wire angle，and getting the welding wire axis perpendicular to the bevel cross section or adopting some thin welding wire.