Abstract: In the process of welding and service for the BOSS welds (The welds of pipe seat connecting the small branch pipe and the main pipe of the nuclear auxiliary system) in nuclear power plants, it is necessary to detect the produced defects are necessary to detect by nondestructive testing. Compared to the commonly used penetrant testing and radiographic testing, phased array ultrasonic testing (PAUT) has the advantages of high detection sensitivity and location accuracy and can be applied to defect detection of BOSS welds. In this paper, PAUT is performed on the BOSS welds of SKETCH14 specification. Appropriate PAUT probes and wedges are chosen in combination with the geometry of BOSS welds to determine the detection scheme. A total of eight ϕ 2 mm side-drilled holes (SDH) and four inner surface slits with 3 mm height were manufactured at the different depths to simulate the volumetric and planar defects and related detection experiments were carried out. The results indicated that the twelve defects can be effectively detected with a measurement error within 6.9% in depth by using the proposed PAUT detection scheme.