Ultra-narrow-gap welding experiment of 1Cr18Ni9Ti austenitic stainless steel has been carried out by the method of constricting arc with ultra-fine granular flux, and microstructures and mechanical property of ultra-narrow-gap welding joint have been measured and analyzed. The results indicate that ultra-narrow-gap welding joint of 1Cr18Ni9Ti austenitic stainless steel takes on equiaxed grains in the backing weld zone and coarse columnar grains in the weld zone of filling bead and cover pass. Equiaxed grain and columnar grain mainly are made of austenite, but there is a small quantity of lath ferrite in the equiaxed grain and columnar grain. Backing weld and filling bead of ultra-narrow-gap welding joint have the similar hardness as base metal, but the hardness of cover pass is lower than that of base metal. The properties of ultra-narrow-gap welding joint are higher than the minimum of corresponding properties of base metal including the tensile strength, yield strength and elongation ratio except reduction of area and impact energy. Besides, corrosion rate of ultra-narrow-gap welding joint is 0.417 g/(m2.h), which is obviously lower than that of base metal