Abstract: Based on the principle and characteristics of ultrasonic tandem scanning, this paper develops a series of tandem scanning devices which are suitable for type K or single-bevel-groove welds of offshore steel structure. The experiment is carried out by using this device, and the results indicate that the device reduces the labor hour investment for this seam examination, improves the inspection accuracy, achieves a stable and rapid detection of surface defects perpendicular to the scan surface (such as incomplete fusion, cracks and other defects of the center).