Abstract: The phased array ultrasonic detection of transverse cracks in weld seam of T23 steel water wall of power plant boilerwas studied through simulation, test block making, process setting, engineering application and verification test. The results showed that ultrasonic phased array detection of such defects had the advantages of fast detection speed, high detection sensitivity and intuitive detection results. When testing, not only the workload of polishing the probe placement surface on the weld was small, but also the cross-construction can be carried out without radiation risk, and the technical advantages were more obvious for projects with tight maintenance period, narrow detection space and high detection sensitivity requirements.