Abstract: The single pass welding of X100 pipeline steel was simulated by Gleeble-3500 thermal simulator at different peak temperatures, and the microstructures and impact toughness of coarse-grained heat-affected zone (peak temperature of 1 300℃), fine-grained zone (peak temperature of 950℃), critical zone (peak temperature of 850℃) and subcritical zone (peak temperature of 650℃) of X100 pipeline steel were studied. The results show that the austenite grains in the coarse-grained zone grew up seriously, and there were massive martensite-austenite (M-A) components at grain boundaries; the impact absorption energy decreased by 42.6% compared with that of the base metal.The grains in the fine-grained zone were completely recrystallized with uniform grain size, and the point M-A components were dispersed in the grain; the impact absorption energy had little loss. The grains in the critical zone were partially recrystallized with different grain size, and the impact absorption energy decreased by 16.4%. The subcritical zone underwent a short time high temperature tempering, and the impact toughness changed little comparing with that of the base metal.