Abstract: The effect of microstructure as a function of welding cooling time (t_8/5) from 800 °C to 500 °C on the impact toughness of coarse grained heat affected zone (CGHAZ) of deposited metal of metal cored wire E120C-K4 was investigated by welding thermal simulation. The results showed that the microstructure of CGHAZ was mainly composed of degenerate upper bainite (DUB) granular bainite (GB) and Acicular ferrite(AF) at t_8/5 from 6 s to 12 s, forming the interlace multiphase microstructure, then the optimal impact toughness was obtained. The microstructure of CGHAZ formed with granular bainite (GB) and Acicular ferrite(AF) and the impact toughness decreased when t_8/5 from 30 s to 120 s. The absorbed energy of CGHAZ was only 24 J at −40 °C at t_8/5 of 120 s, the worst impact toughness. The key of improving impact toughness at t_8/5 from 6 s to 12 s was: ① forming the interlace multiphase microstructure; ② refining grains; ③ more high-angle grain boundaries per unit distance.