Abstract: With the great increase in the welding demand of bridge structures, the welding technology of steel has developed rapidly.Q345qD steel, one of the bridge steels, has attracted much attention owing to its high strength, good toughness and excellent weldability.The T-joint of Q345qD steel was welded by MIG welding, the microstructure transformation was observed and the influence of welding temperature field on the structure was analyzed.The results show that the weld is well formed.The microstructure of the welded joint is mainly composed of ferrite and pearlite.The content of pearlite in the bottom layer is relatively high, but relatively lower in the filling layer.Fine ferrite and pearlite are observed in the heat-affected zone accompanied by the generation of fine bainite.According to the thermal cycle curve of the joint, obtained by finite element simulation, the cooling rate of the bottom layer is fast, and the temperature can reach 467 ℃ after two times post-heat treatments, which promotes the generation of upper bainite.The temperature of the capping layer rapidly heats up and then cools down owing to the effect of the residual temperature of the previous layer.The temperature of the filling layer and the heat-affected zone exceeds the AC3 temperature and undergoes a normalizing process under the heat effect of the previous welding process.Highlights: In this paper, the finite element method is used to simulate the temperature field of the T-shape Q345qD joint during multilayer welding.The phase transition process of the welded joint is revealed by microstructure analysis and welding temperature field change of the joint.