Abstract: Single and multiple thermal cycles of welded joint heat-affected zone(HAZ) for NiCrMoV 980 MPa low carbon bainite high strength steel are simulated by welding thermal simulation technique.The impact properties of HAZ are studied at room temperature and -50℃.Based on observations of microstructures and fracture surfaces for various HAZ,the experimental results indicate that the impact property of coarse grain zone is the best,and the impact property of fine grain zone is the worst.So the fine grain zone is the weakest part of the welded joint HAZ.The second heat cycle of coarse grain zone makes Charpy properties increase,and the second heat cycle of fine grain zone makes Charpy properties decrease.The coarse lath bainite dominates the microstructure of coarse grain zone,and large and deep dimples are integrated and connected more closely in the fracture surface of coarse grain zone.Twin martensite dominates the microstructure of fine grain zone.Small and shallow dimples are connected,and a small number of larger dimples are existed in the fracture surface of fine grain zone.