Abstract: Narrow gap automatic argon arc welding machine is used to weld a large-diameter thick-walled G115 steel pipe. The microstructures of the welded joint is observed by optical microscope. The mechanical properties of the joint are tested and analyzed by tensile, bending, impact, hardness, and residual stress tests. The results show: The microstructure of the narrow gap argon arc welded joint is tempered martensite. There is no micro cracks in the heataffected zone which width is only 1.7mm. The joint has good strength, with the bending test satisfied the standard requirements, excellent impact test results. Maximum hardness value is 249HV. Which satisfied the standard requirements. The maximum residual stress in the welded joint is located at the center of the weld, with a tensile stress of 278MPa, which was 52% of the yield strength of the base metal. The research of full-position narrow gap automatic argon arc welding process laid a solid technical foundation for the engineering application of G115.