Abstract: The health of the pressure boundary of nuclear power plant operation affects the safety and economy of NPP directly. Pressure vessels and pipelines may have manufacturing defects such as pores, slag inclusion during manufacturing, or certain allowable defects are found in regular in-service inspection. It is of great significance to detect, track and evaluate these defects. In this paper, the localization algorithm and device development of defects in the pressure boundary by ultrasonic guided wave technology are studied and applied. As an auxiliary means of in-service inspection, this method has very important guiding significance for the pressure vessels and evaluation of their continuous operation.