Abstract: In the nuclear power plant system, steam turbine and water conveyance system work under high pressure and the operating conditions are relatively harsh. Considering the operation safety of nuclear power plant, it is necessary to check the welding seam of the outer branch pipe and manhole gate of the high pressure exhaust line in order to find the defects and deal with them in time. Following a comparison on various testing methods, the phased array ultrasonic testing technology was established and applied to the field inspection. Many oversized defects were detected and dealt with in time, and the hidden trouble was therefore eliminated. Eventually, the above-mentioned measures ensure the safe operation of the power plant.