Abstract: The defects of the P92 material steam pipe weld were found by conventional ultrasonic inspection in an ultra-supercritical power station, and the defects were reviewed by phased array inspection. The static waveform and dynamic waveform characteristics of the defect are analyzed, and the nature of the defect is qualitatively judged. Finally, the whole welding seam is turned by external clamp ring groove machine, which verifies the correctness of the detection conclusion. The welding process and its possible problems in the execution are analyzed, and the causes of the defects are proposed. According to this, the process control of the repair welding is carried out. In this paper, the degree of damage of weld slag defects is also discussed. It is proposed that the possibility of cracks caused by slag inclusion in alkaline electrode is lower than that of acid electrode.