Abstract: There is an error in the defect length by the zone discrimination strip pattern measurement in the Automatic Ultrasonic Testing (AUT) of the girth weld of the subsea pipeline. The artificial reflectors of known sizes in the test block can be used to decrease the error by using different methods and by the comparison of related error values. Through data analysis, it is proved that the defect length corrected by the test block error value is more accurate. It plays an active role in the assessment of defects close to the critical size allowed by the standard, which can reduce the misjudging and undetected defect caused by the defect length assessment to some extent and improve the construction efficiency.