Abstract: The effect of cooling rate on the microstructure and properties of coarse grain heat affected zone (CGHAZ) of high strength steel were analysed by thermal simulator, optical microscope (OM), scanning electron microscope (SEM), electron backscatter diffraction(EBSD), Vickers hardness test and Charpy V-notch impact test.The results show that when the cooling rate is 0.5-4 ℃/s, the microstructure of the CGHAZ is mainly granular bainite.When the cooling rate is 6-20 ℃/s, the microstructure of the CGHAZ is mainly lath bainite.When the cooling rate is 30-60 ℃/s, the microstructure of the CGHAZ is martensite.The measured value of the martensite transformation initiation temperature M_s is 437 ℃.When the cooling rate is 1℃/s, the impact energy at room temperature corresponding to the CGHAZ is 4.9 J, and the proportion of high angle grain boundaries (HAGBs) is 28.6%, and the fracture type is quasi-cleavage fracture.When the cooling rate is 10, 60 ℃/s, the impact energy at room temperature corresponding to the CGHAZ is 20.2, 18.9 J respectively, and the proportion of HAGBs is 41.4%, 31.4% respectively, the fracture type is ductile fracture.In summary, the toughness is the worst when the microstructure of the CGHAZ is granular bainite, and the toughness is the best when it is lath bainite.