Abstract: The microstructure and toughness of the coarse-grained welding heat-affected zone(CGHAZ) of 1 000 MPa low carbon QT steel with different heat input were investigated by thermal simulation with a Gleeble-1500 thermal simulator.The results show that the impact toughness firstly increases and then decreases and the microstructure varies from martensite to duplex structure of matensite and bainite with the welding heat input being increased in the first welding thermal cycle.The decrease of the toughness was caused by the presence of carbide in coarse lath martensite and bainite.All the effect of the second thermal cycle with 900℃ peak temperature on toughness is not improved because of the inheritance of grain and microstructure.There is an inheritance of grain and microstructure at low welding heat input,and there is only an inheritance of grain at high welding heat input.The main reason to cause the decrease of the toughness is the formation of the coarse bainite and precipitation carbide in the second thermal cycle at high welding heat input.