Abstract: Boss welds (Branch pipe fillet welds) were widely used in nuclear power pipeline systems, and many defects were found during pre/in-service inspections of the welds. At present, there are some limitations in the radiation and penetration testing. In this paper, we developed a phased array inspection process by combining the structure and processing of the typical BOSS welds in a nuclear power plant. The test was carried out on the artificial defect test block and on-site BOSS welds of the nuclear power plant, and the results were compared with the X-ray examination results. It was found that the developed phased array ultrasonic detection technology could effectively detect the inner surface defects of BOSS weld in nuclear power plant. The results of field application are different from those of radiographic detection, but the larger defects can be detected.