Abstract: Ultrasonic phased array and routine ultrasonic technology were applied in the detection of the butt welds of high-pressure nitrogen vessels in Baling Company of SINOPEC, which had abnormal thickness and could not be stopped for detection. As compared with ultrasonic phased array, five serious defects were undetected by routine ultrasonic, and the length and depth of defect detected by routine ultrasonic was longer and deeper, and the workload of routine ultrasonic was four times as much as ultrasonic phased array. It can be concluded that ultrasonic phased array technology has obvious advantages in defect detection, reliability and efficiency.