Abstract: Ultra-narrow-gap welding with constricted arc by ultra-fine granular flux was used to weld 1Cr18Ni9Ti austenitic stainless steel,and weld formation was studied at different welding speeds, arc voltages and welding currents. The results indicate that at the heat input of 1.75 kJ/mm and depth-to-width ratio of 1.34, pear-shaped bead cracking does not easily create, and filling height by molten wire in ultra-narrow gap can reach up to 11.5 mm in single-pass welding. At the other constant parameters, with the increase of arc voltage, ultra-narrow-gap weld formations of 1Cr18Ni9Ti austenitic stainless steel shows "convex weld", "concave weld" and "arc climbing up along the sidewalls from the gap bottom", respectively. Voltage range being fit for ultra-narrow-gap welding of 1Cr18Ni9Ti austenitic stainless steel with constricted arc by ultra-fine granular flux is 26-32 V at the current range of 200-320 A.