Abstract: It is difficult to detect transverse crack in weld seam of high temperature superheater header in a power plant with traditional angle probe scan on both side of weld seam. To solve the problem, the method of circumferential detection was used to detect the weld seam. The defect echo identifying method and the influencing factors of circumferential detection method were analyzed and discussed. The result indicated that it was effective to test transverse crack in welding line.