Abstract: The microstructure and impact toughness of heat-affected zone (HAZ) of X100 pipeline steel under the conditions of different welding heat inputs and secondary heat cycle peak temperature were studied by welding thermal simulation. The results show that the microstructure of HAZ mainly composed of lath bainite when one cycle welding heat input was 15 kJ·cm-1, and the microstructure of HAZ had excellent impact toughness. When the heat input was 25 kJ·cm-1, the microstructure of HAZ mainly consisted of granular bainite and impact toughness drastically decreased. For two cycles welding, the HAZ had good impact toughness when the welding heat input was 15 kJ·cm-1 and secondary heat cycle peak temperature was 600 ℃. However, the microstructure of HAZ were coarse granular bainite with the increase of secondary heat cycle peak temperature, and the impact toughness quickly reduced. When the heat input was 25 kJ·cm-1, the impact toughness was very poor because the microstructure of HAZ was mainly granular bainite.