Abstract: The thermal cycle process of the heat affected zone (HAZ) of FB780 steel welded joint under single pass welding condition was simulated by the Gleeble-3500 thermomechanical simulator. The microstructures of HAZ at different welding peak temperatures and for different t_8/5 times (cooling time from 800℃ to 500℃) were studied, and the micro-hardness and impact toughness of HAZ under different welding conditions were tested. The results show that when the t_8/5 was the same, the microstructure of HAZ coarsened with the increase of peak temperature. When the peak temperature was constant, with the increase of t_8/5, the ferrite lath widened and gradually intergrated; the M/A component coarsened; the coarse ferrite grains were mixed with the fine quasi-polygonal ferrite grains; the metallographic homogeneity deteriorated. At the same peak temperature, the hardness and impact energy of HAZ decreased with the increase of t_8/5. At the peak temperature of 1 175℃ and t_8/5 of 6 s, the impact fracture morphology of HAZ at low temperature (-20℃) showed dimple pattern, and the dimple was relatively large; the toughness of HAZ at low temperature matched with that of the base metal; the hardness was relatively high; all indicated that the process was the best single pass welding process.