The microstructure of heat affected zone(HAZ)has a great effect on the mechanical properties of steel welding. In order to study the evolution of the microstructure and properties of titanium microalloyed steel Q345B in heat affected zone under different welding line energy, an experimental study on the microstructure evolution of Ticontaining microalloyed steel Q345B in heat affected zone during welding process was carried out with a Gleeble 3500 thermal simulator. The effects of cooling rate on the microstructure and impact toughness of heat affected zone were also analyzed. The results revealed that the microstructure of heat affected zone mainly consisted granular bainite. When t8/5 was 120 s, the impact toughness in heat affected zone was improved with the occurrence of acicular ferrite.