Abstract: Pipeline rupture resulted from welding defects in Daqing-Tieling Oil Transport Pipeline greatly affected oil transport production. The article analyzed in detail the reason of the rupture and examined the ruptured pipeline section. And pointed out that the major reason of the accident was caused by offset welding in inner weld on spiral welded seams and there were a larger incomplete fusion zone on one side on the bevel. In case of slight higher operation pressure, the welds with defect as be unable to bear the working pressure and caused the pipeline failure consequently. The analysis on the reason of the above accident will be of a major directive significance in ensuring oil transport pipeline's safe operation.