Abstract: A type of Mn-Si-Ni-Cr system solid welding wire was developed for a new type of marine 440 MPa grade high strength low alloy (HSLA) steel. The steel was welded by metal active gas arc welding with 11.5, 16.5 and 21.5 kJ/cm heat input, while the effect of heat inputs on the microstructure and properties of the weld seams was mainly studied by means of optical microscopy, scanning electron microscopy and transmission electron microscopy. The results show that with the increasing heat input, the weld seam microstructure changes from mainly granular bainite and lath bainite to mainly acicular ferrite, and then to mainly acicular ferrite, ferrite side paltes and proeutectoid ferrite, while the content of M-A constituent, the proportion of diameter greater than 1 μm of inclusions and the average diameter of inclusions in weld seam gradually increase; with the increasing heat input, the hardness of the weld seam continuously decreases, and the impact toughness at the weld seam, fusion line, and fusion line +2 mm first increases and then decreases. Meanwhile the corrosion resistance of the weld seam also increases and then decreases; The tensile specimens of three types of welded joints are all fractured at the base metal, the bending specimens are all intact.