This paper compares and analyzes several methods of butt welding of 9% Ni steel vertical weld in large LNG tank,and introduces the advantages of TIP-TIG and pendulum welding technology in DPS-3G automatic welding system. It is considered that DPS-3G automatic AC gas tungsten arc welding technology can completely solve the industry problem of side wall incomplete fusion of 9% Ni steel narrow groove welding. At the same time,the low temperature impact toughness of-196 ℃ is excellent,the performance and quality of welding joint are reliable,the first pass rate of non-dest-ructive testing is high,high efficiency welding is realized,the construction period is greatly shortened,and the construction cost is saved.