Abstract: Under corrosion environment, the flange is susceptible of corrosion defects on the sealing surface resulting to the leakage, thus affect the safety of production and leading to harmful consequences. In this paper, by making use of the features of ultrasonic phased array detection technology, i.e. the variable beam angle and the advantages of dynamic focusing, suitable for complex structures and real-time imaging, etc., the ultrasound phased array testing procedures targeted for the flange was determined through structural analysis, sound field simulation, experimental research and the applications of field testing. Effective detection of corrosion defect on the sealing surfaces from the flange external side was achieved without disassembling it.