Abstract: The effect of heat input on microstructure and impact property of heat affected zone (HAZ) of crack-free steel plate Q500CF was examined by using submerged arc welding (SAW) test and Gleeble simulation. It is found that,SAW joint show excellent impact property in HAZ (A_KV,-20℃≥146 J) with heat input between 15 and 50 kJ/cm. Among HAZ,fusion boundary (FL) have the lowest impact value,which decreases with heat input increases. Coarse grained HAZ microstructure of SAW joint changed from a mixture of lath bainite (LB) and granular bainite (GB) at heat input of 15 kJ/cm to a single GB structure at heat input of 50 kJ/cm. A large amount of 1~7 μm M-A constituents are formed at prior austenite grain boundary at inter-critical coarse grained HAZ,resulting in impact toughness deterioration of this region. Gleeble simulation results show that higher heat input of 50~70 kJ/cm causes coarsening of GB, which decreases impact toughness of coarse grained HAZ. In conclusion,heat input should be restricted to 15~50 kJ/cm for sake of impact property of HAZ.