Abstract: In this paper, an ultrasonic phased array inspection method was proposed to inspect the corrosion pits in the inner surface of pressure vessel without shut down. The models of hemisphere pitting and flat pitting corrosion with different inner surface sizes were established by the software CIVA. Then, the acoustic field was simulated by virtue of conventional ultrasonic inspection and ultrasonic phased array inspection, respectively. Finally, the defects were identified by means of bottom wave reduction method and defect echo method. It is indicated that the conventional ultrasonic inspection cannot effectively inspect the corrosion pits in inner surfaces while ultrasonic phased array inspection can fulfill the inspection of corrosion pits in inner surfaces because of the focusing function of the wafer in its probe. The ultrasonic phased array inspection is much superior to the conventional ultrasonic inspection. In addition, based on C scanning imagery, the ultrasonic phased array inspection can quantity and locate the pits precisely. It provides a useful reference for the inspection of corrosion pits in inner surfaces of pressure vessels without opening.