Abstract: In order to obtain the optimal heat treatment process for offshore FH790 heavy plate, four cooling methods, namely water cooling, oil cooling, air cooling, and sand cooling, were used to represent different cooling rates to study and analyze the effect of different quenching cooling rates on the microstructure and mechanical properties of FH790 offshore steel. The analysis results show that the microstructure of experimental steel under water cooling and oil cooling is bath martensite. The microstructure of the experimental steel under air cooling is bath martensite mixed with granular bainite, and granular bainite and ferrite generated under sand cooling. As the cooling rate decreases, the strength of the experimental steel gradually decreases, and the elongation does not change much. The -60 ℃ low-temperature impact toughness is highest under oil cooling, with an average impact energy of 150 J. The strength is high under water-cooling, but the toughness is poor. The strength, plasticity, and toughness of air cooled and sand cooled experimental steels are relatively poor. Therefore, to ensure the uniformity of mechanical properties at different positions in the thickness direction of the FH790 offshore heavy plate, it is required that the cooling rate at the core position be greater than that of the oil cooling method by 40 ℃/s during quenching, so as to obtain the ideal microstructure of the core and improve the low-temperature impact toughness of the FH790 offshore heavy plate core.