Heavy steel plates (max thickness， 80 mm) for offshore platform with yield-strength greater than 500 MPa have been produced by thermomechanically controlled process (TMCP). Continuous cooling transformation in the simulated coarse-grained heat-affected zone (CGHAZ) was investigated. The influence of heat input[E]on the microstructure and mechanical properties of the CGHAZ with and without second reheating peak temperature[(t2p)]was studied by means of metallographic analysis， hardness testing， instrumented impact testing and electron backscatter diffraction (EBSD). The microstructure of the CGHAZ changes from lath-like Bainite (LB) to granular Bainite (GB) as[E]increases， resulting in decreased hardness， impact energy， and the fraction of large-angle grain boundaries (＞15°). The predominant microstructure of the CGHAZ is refined LB when[E]is lower than 50 kJ/cm. Coarsened martensite-anstenite constituents (MA) are formed in the intercritically (reheatedCGHAZ) with[t2p]at 750 ℃. This leads to decreased impact toughness showing brittle fracture regardless off the variation of heat input. When[E]is smaller than 50 kJ/cm and[t2p]is higher than 850 ℃， the fraction of high angle grain boundaries increases and the impact toughness is largely improved showing ductile fracture.