Abstract: 12 mm thick Q345 E steel plate was welded by traditional TIG welding process and hot wire TIG welding process,respectively.Metallographic analysis and mechanical properties testing such as tensile test,hardness test and impact test at-40 ℃ had been conducted.The results showed that the microstructures and grain sizes of both welded joints were basically the same. The microstructures of weld are composed of ferrite and pearlite,and a small amount of carbon-free bainite and granular bainite are found in the upper weld. The microstructures of the heat affected zone are consisted of ferrite and pearlite and there are a few granular bainite in the coarse grained zone. The results of mechanical properties testing showed that the welded joints of both welding processes fracture at the base metal,and the tensile strength is between 530 MPa and 545 MPa. The hardness distribution of the welded joints is similar,the center of the cosmetic weld has the highest hardness with a value of 221 HV10 which is lower than 350 HV10. All of the impact energy values of the welded joint are higher than 34 J,and the fracture morphology is dimple plastic fracture. All the mechanical properties of strength,hardness and impact toughness of the welded joints meet the demands. There were no obvious difference between the microstructures and mechanical properties of the hot wire TIG welded joint and the traditional TIG welded joint at the same arc power,and the welding efficiency of hot wire TIG is 1. 6 ～ 2 times of that of traditional TIG welding.