Abstract: The main cause for the hydrogen-introduced weld defect of the LSAW pipe is analyzed.As a result,it is identified that the main cause is that some water or oil enters the welding pool.Accordingly it is emphasized that preventing any water or oil from entering the welding pool is the critical measure to prevent the said defect.Possible channels for water or oil to get into the welding pool are summarized.And suggestions are proposed to prevent the trouble.