Abstract: The formation and mechanical properties of welded joint for 400MPa ultra-fine grain steel were studied based on surface tension transfer technology, CO₂ arc welding, special narrow groove and different heat input.The results showed that the welded joint with good fusion, one side welding with back formation and narrow HAZ (heat-affected zone)(about 1 mm)was obtained with 3-4 kJ/cm heat input.The hardness and tensile strength of the joint were higher than those of the base metal.The embrittlement and softening of HAZ was not found, and the bending plasticity was qualified.The impact toughness of HAZ was about 60% higher than that of the base metal, which can be caused by the granular pearlite transition, multi- phase non-equilibrium microstructure and higher yield stress.