Abstract: The influence of welding heat input on the microstructure characterization and properties of coarse grain heat-affected zone(CGHAZ) in a X100 pipeline steel were investigated by means of thermal simulation technique, microscopic analysis method and mechanical property testing. The results showed that the strength and toughness of X100 pipeline steel decreased with the welding heat input increasing. The microstructure of CGHAZ was mainly made up of bainitic ferrit and granular bainitic which could bring excellent strength and toughness when welding heat was about 10 kJ/cm. The quasi-polygonal ferrite and granular bainitic were formed with welding heat input about 20 kJ/cm, which could get fine strength-toughness. When the welding heat input was higher than 30 kJ/cm, the strength and toughness decreased because of the increasing of polygonal ferrite. Therefore, welding heat input at range of 10-20 kJ/cm was recommended in the welding process of X100 pipeline steels.