The microstructure and impact properties of Q345 E low alloy steel in the superheated zone under different welding heat input were studied by welding thermal simulation technology. The results indicate that with the increase of welding heat input,the microstructure of the superheated zone is gradually transformed from pro-eutectoid ferrite+pearlite and part of martensite into granular bainite and acicular ferrite structure. When the welding heat input is 3.06 kJ/mm,a small amount of Wei’s tissue appears,with the lowest impact energy. As the weld heat input increases,the average hardness value of the coarse grain zone gradually decreases. When the welding heat input is 1.35 J/mm,there is a small amount of lath martensite in the microstructure,and the hardness is the largest. When the welding heat input reaches 2.68 kJ/mm,acicular ferrite appears in the microstructure at this time,and the hardness decreases more obviously. The overall impact energy is gradually decreasing. When the welding heat input is 2.68 kJ/mm,the microstructure was acicular ferrite and the impact energy is the largest.