In view of the problems of low efficiency，poor consistency，difficult result tracing and safety risk of climbing operation in the phased array ultrasonic detection of the long tube section of the new launch vehicle，the long tube section is selected as the application object in this paper.the research on ultrasonic in-situ automatic detection technology of longitudinal seam in friction stir welding of long tube section of tank is carried out，which is verified by the design and development of automatic detection system and automatic test.The results show that the detection system breaks through the on-site automatic detection technology，and satisfies the synchronous automatic scanning of the longitudinal seam of the cylinder section with a height of 0 to 7.5 m，and the sensitivity reaches Φ 0.4 mm equivalent hole，which realizes the efficient，safe and accurate detection of the internal quality of the longitudinal seam of the long tube section.