Abstract: Taking the Φ1 219 mm×18.4 mm as an example,the UT waveforms concerning the two types of harmful defects of the weld of the large-sized heavy-wall HSAW pipe,i.e.,the crack and the non-penetration welding are described here in the essay.Also studied are the regularities of defect positioning,quantification and qualification by the UT.Thanks to the NDT to the weld defects,pinpoint preventative,controlling and adjusting actions can be timely taken to direct the pipe formation and welding processes so as to avoid as far as possible occurrence of the harmful defects,and thus enhance the primary qualification rate of the weld,and instruct without any delay the repair of the weld.