Abstract: Using phased array ultrasonic testing method, this paper, through the analysis of the acoustic characteristics simulation, block design and validation test, the phased array ultrasonic testing of 304 L stainless steel thin-wall weld of small tube is under study, results of on-site verification show that for incomplete fusion area type defects such as cracks, incomplete fusion, the detection rate of phased array ultrasonic and X-ray detection is 100%.For volumetric defects, the phased array detection ability is similar to that of radiographic detection. For small defects about 0.5 mm, there is a certain deviation in reflected wave amplitude due to the influence of defect morphology and properties. In addition, the detection accessibility of PAUT is better than that of radiographic detection, and its detection efficiency is higher than that of radiographic detection. Moreover, alternating construction can be carried out, and the result data can be reviewed with good data repeatability and no radiation risk.