Abstract: The thermal simulation and microstructure a-nalysis technology were used to investigate the microstructure and mechanical properties of the heat affected zone for X80 pipeline.Results indicated that the toughness of coarse-grain zone and crit-ical heat affected zone in X80 pipeline HAZ were weak.Brittle zone happens in coarse-grain zone.The low toughness of coarse-zone was ascribed to the coarse grain size,the increasing quanti-ty of granular bainite and upper bainite as well as other unbal-ance low and middle temperature transformation products.The impact toughness of coarse-grain zone decreases with the increase of t_8/5 Fined-grain zone is the softening zone in HAZ,the sof-tening degree increases with the increase of heat input.