Abstract: The effect of heat input of 14-20 kJ/cm on the microstructure and mechanical properties of the 18 mm thick 800 MPa grade HSLA steel MIG welded joints was investigated by OM, SEM, EBSD as well as tensile and impact tests.The results indicate that the microstructure of the weld metals under different heat inputs is primarily composed of a mixture of bainite and martensite.The microstructure of the coarse grained heat affected zone (CGHAZ) is primarily composed of lath martensite and coalesced bainite.With the increase of heat input, the proportion of granular bainite increases, and the lath substructure changes from slender and parallel strips to coarse blocks, and the impact toughness of the weld metal firstly increases and then decreases.When the heat input is 17 kJ/cm, the short strip lath bainite, lath martensite and fine granular bainite in the weld metal microstructure are interlocked and evenly distributed, which makes the the weld metal have high thoughness and small dispersity.Under the heat input of 14-20 kJ/cm, the minimum value of the impact energy of the weld metal, the fusion line, the heat affected zone at-50 ℃is 105, 190, 166 J, respectively.The tensile strength of the weld metal has little difference with that of the base metal, and the welding material and the base metal are well matched, and the welded joints with good strength and toughness matching are obtained.