Abstract: The influence by the change of thermal cycles on the impact toughness of HAZ in welding and its fracture characteristics are studied in this paper. Analysis on the fracture of impact specimens was carried out by SEM.The experimental results illustrated that: the higher is the cooling rate, the better will the impact toughness be. The main cause of low impact toughness is that following the long cooling time, the structure becomes coarsing and appears corse-grained twin-mastensite. After simulative heating and twice progressive heating at 900℃ and 600℃, the impact toughness of HAZ might be improved.