Abstract: The difficulties faced by ultrasonic testing of the liquid ammonia tank sealing head weld were analyzed and the phased array ultrasonic testing technology was used to improve the reliability of defects detection. The prefabricated defects were successfully detected, and the feasibility of the detection process was also verified. The designed test process was applied to the site inspection and the results showed that phased array ultrasonic testing had the advantages of intuitive defect image display and high detecting reliability.