Abstract: The effects of technology parameters of simulated welding thermal cycle(cooling time t_8/5)on microstructures and impact toughness of CGHAZ in micro-colcium steel were studied.The M-A constituents in granular bainite were etched by Lepera reagent.The pattern, quantity and distribution of M -A constituents with different t_8/5 were studied in optical microscope and TEM.The research shows that when the cooling rate is lower the M-A constituents are bacillary and directional, when the cooling rate is higher the M-A constituents are granular and direction-free.When t_8/5=40 s the impact toughness is better than other cooling time.