The automation of vertical seam welding of 9%Ni steel for large LNG storage tank is an urgent industry problem to be solved. In this paper, GMAW-P automatic welding technology is used to weld the vertical seam of LNG storage tank. In order to ensure the quality and reliability of vertical welded joint, the microstructure and mechanical properties of welded joint were studied by microstructure observation, tensile test, bending test, impact test and hardness test. The results show that the weld formation of 9% Ni steel automatic vertical welding joint is good and the microstructure transition is uniform. The tensile properties of the weld metal are excellent. There is no crack in the bending test of the joint.The low temperature toughness of the-196 ℃ impact test is good. The mechanical properties are qualified according to the requirements of welding procedure qualification standard, which provides certain theoretical guidance for the application and promotion of high-efficiency automatic vertical welding technology for large LNG storage tanks.