Abstract: The welding defects such as crack, non-fusion and non-penetration in the butt welding joint of the small pipe were detected by phased array ultrasound and compared with the results of X-ray detection.The method of interpretation of the phased array ultrasonic detection data is summarized through the field test data, which can be used as reference for the identification and evaluation of the results of the phased array ultrasonic images of small diameter pipe.