Abstract: Thermal simulation tests were conducted on Q690 steel at cooling rates of 0.1–100 ℃ • s⁻¹. By the combination of metallographic analysis and hardness testing, the welding continuous cooling transition curves of test steel was drawn, and the microstructure transformation law of test steel was clarified. According to the welding continuous cooling transition curves, the heat input of the root pass was determined as 15 kJ • cm⁻¹, and that of the fill and cover passes was determined as 13, 18, and 22 kJ • cm⁻¹. The metal active gas (MAG) butt welding test of 30 mm thick Q690 steel was carried out under the above heat input conditions, and the influence of heat input on the microstructure and mechanical properties of the joint was studied. The results show that there were no cracks, pores, or other defects in the joint under different heat inputs, and the welds were well formed. The microstructure of weld mainly consisted of bainite. The microstructure of overheated and normalized zones in the heat affected zone was composed of bainite and martensite, and that of the incomplete normalized zone consisted of bainite, martensite, ferrite and pearlite. With the increase of heat input, the size of grains in the overheated and normalized zones increased, and the tensile strength of the joint first increased and then decreased. When the heat input was no more than 18 kJ • cm⁻¹, the tensile strength met the usage requirement (not less than 770 MPa). Under different heat inputs, there were no cracks in the joint after 180° bending. The impact absorption energy at −20 ℃ in the heat affected zone and weld met the technical requirement (not less than 47 J). The hardness of the weld was the lowest, and the hardness of the heat affected zone was the highest. When the heat input was 13, 18 kJ • cm⁻¹, the hardness of the heat affected zone was much higher than that of weld and base metal, When the heat input was 22 kJ • cm⁻¹, the hardness of the heat affected zone was only slightly higher than that of base metal and weld.