Abstract: It is needed to localize the flaw position for echoes over the gate on TOFD longitudinal welds testing.The size of the defect was characterized by the length and height, and the location of the defect was characterized by its depth and horizontal distance.It was considered to make the main beam focusing on about two-thirds of the partition of the weld under test and the defect depth calculation formula was based on planar weld.The above-mentioned method can help inspection personnel to accurately judge defect position in the cylinder surface longitudinal weld TOFD test, and to improve the efficiency of inspection while ensuring accuracy.