Abstract: The influence of the heat-input on microstructures and property in regions of simulated welding heat-affected zone(HAZ) of 06CuNiCrMoNb steel were investigated and the relationship between the toughness of the coarse grain heat-affected zone(CGHAZ) and microstructures were discussed.The results show that the tensile strength of simulated HAZ dose not debase than that of mother metal,but the low-temperature impact toughness of the CGHAZ declines rapidly on condition of heat-input above 30kJ/cm.The TEM observation indicates the shape of bainite-ferrite is thin lathlike and there are residual austenite films between lath,while with the increase of heat-input,the quantity of block ferrite increases and M-A constituent appears in ferrite matrix.