The Q620qENH steel plates were welded via a Gleeble-3800 thermal simulation set, while the welding thermal cycles of varying heat inputs ranging from 10 kJ/cm to 80 kJ/cm were applied. Then the steel and its weld joints were characterized by means of optical microscopy (OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), Vickers hardness tester, and low-temperature impact tests, in terms of the effect of heat input on microstructure and mechanical properties of the coarse-grained heat-affected zones (CGHAZ) of weld joints. The results reveal that at lower heat inputs (10 kJ/cm to 40 kJ/cm), the microstructure of the CGHAZs is primarily composed of lath bainite (LB) and a minor fraction of granular bainite (GB), with microhardness values stabilized between 310 and 330HV, and with impact energy at -40 ^oC maintained within the range of 250 J to 300 J. The impact fracture surfaces exhibit a pattern of multi-scale reticular dimples feature. As heat input increases from 50 kJ/cm to 80 kJ/cm, the GB content increases while LB diminishes, a pronounced reduction in the lath bundle structure, significant coarsening of the microstructure, enlargement of the prior austenite grain boundary size, and increased proportion of M/A island constituents, which results in a decrease in microhardness to 253HV. Upon further increasing the heat input from 60 kJ/cm to 80 kJ/cm, there is a significant deterioration in the low-temperature toughness of the steel, with the impact energy at -40 ^oC plummeting from 273 J to 36 J. The impact fracture surfaces display a distinct river-like pattern, indicative of brittle fracture characteristics. This is attributed to the fact that the transition from ductility to brittleness of this grade steel occurs at a heat input of 60 kJ/cm. This is attributed to joints the fact that the transition from ductile-to-brittle of the steel occurs at a heat input of 60 kJ/cm. Consequently, the optimal heat input range for the Q620qENH steel is below 60 kJ/cm to achieve a favorable matching between high hardness and excellent low-temperature toughness.