Abstract: The effects of different welding heat input on microstructure and mechanical properties of heat affected zone in X100 pipeline steel was studied by means of welding thermal simulation technology,microstructure analysis and impact test.The results showed that when the heat input is greater than 45 kJ/cm,the microstructure is mainly composed of granular bainite,and M/A island is irregular large block with sharp corners,leading to the decrease of material toughness; when the heat input is 15~35 kJ/cm,the microstructures is mainly composed of lath bainite and a small amount of acicular ferrite,with fine M/A island distributed in the ferrite matrix,which does not deteriorate on the toughness,and the resulted impact absorbed energy is at the highest value.When the heat input is less than 15 kJ/cm,martensite appears in microstructure,the hardness of HAZ is the highest but the toughness decreases.