Abstract: At present, in domestic thermal power units, components such as low-temperature reheaters and high-temperature reheaters often use a structure with sealing sleeves installed at the top of the roof, which are welded to the heating surface pipe to form sleeve welds. Due to the presence of root gaps, stress concentration, and the complex temperature environment and thermal expansion force, cracks are prone to occur at the welding seam of the casing, which can extend to the heating surface tube, seriously affecting the operational safety of the unit. There is currently no suitable means to effectively detect cracks at this location for the casing weld seam. Based on theoretical analysis, computer simulation technology was used to guide the development of ultrasonic phased array detection technology, achieving acoustic beam coverage of the casing weld seam, effectively solving this problem and providing some reference for on-site inspection.