Abstract: Thermal cycle process of welding coarse-grained region of Q690 high strength bainite steel under the single bead welding condition was simulated by using thermomechanical simulator. Microstructure, impact toughness, fracture morphology and microhardness of coarse grained region at different cooling times (t8/5) were studied. The results show that when t8/5≤10 s, coarse-grained region with relatively high impact toughness consisted of lath bainite and granular bainite, and fine flaky and granular M/A islands with relatively small amount were found; When t8/5＞10 s, microstructure of coarse-grained region evolved into granular bainite, the size and content of M/A islands increased obviously and changed into coarse block and island morphology, and the impact toughness was severey worsened.