The　effect　of　different　heat　inputs　of　1.45　kJ/mm,　1.78　kJ/mm　and　2.31　kJ/mm　on　the　microstructure　and　mechanical　properties　of　deposited　metals　of　the　self-developed　AWS　A5.28　E120C-K4　high　strength　steel　flux-cored　wire　was　studied　by　optical　microscope,　scanning　electron　microscope　and　mechanical　property　test.　With　the　increase　in　heat　input,　the　results　showed　that　the　microstructure　of　deposited　metals　became　coarse.　Acicular　ferrite　increased　at　first　and　then　decreased,　granular　bainite　increased　and　degenerated　upper　bainite　and　martensite　decreased　slightly.　Under　the　low　heat　input　of　1.45　kJ/mm,　the　cooling　rate　was　fast　and　the　element　diffusion　was　uneven,　which　caused　composition　segregation　and　easy　to　form　large　size　inclusions　SiO2-TiC-CeAlO3　with　weak　binding　to　the　matrix.　Under　the　middle　heat　input　of　1.78　kJ/mm,　the　composite　rare　earth　inclusions　in　dimples　were　mainly　TiC-CeAlO3.　The　dimples　were　small　and　uniformly　distributed,　and　the　dimple　fracture　mainly　depended　on　the　wall-breaking　connection　between　medium-sized　dimples　rather　than　an　intermediate　media.　Under　the　high　heat　input　of　2.31　kJ/mm,　SiO2　was　easy　to　adhere　to　high　melting　point　Al2O3　oxides　to　form　irregular　composite　inclusions.　Such　irregular　inclusions　do　not　need　to　absorb　too　much　energy　to　form　necking.　Finally,　the　integrated　effects　of　microstructure　and　inclusions　resulted　in　the　optimum　mechanical　properties　of　deposited　metals　with　a　heat　input　of　1.78　kJ/mm,　which　was　a　tensile　strength　of　793　MPa　and　an　average　impact　toughness　at　-40　degrees　C　of　56　J.