Abstract: In order to evaluate the mechanical properties of welded joint of thick Q345B steel plate, 67 mm thick plates was welded using MAG welding and submerged arc welding process, respectively, and the microstructure and mechanical properties of the welded joints were tested including tensile, bending, room temperature impact and low temperature impact test. The results show that the welded joint obtained in the welding process has good appearance and the defects are few, and the microstructure of SAW weld is coarse columnar grains of the first eutectoid ferrite and acicular ferrite. MAG weld is composed of first eutectoid ferrite and acicular pearlite, and microstructure of SAW weld interlayer is more uniform, consisting the lamellar pearlite and ferrite. The average yield strength and tensile strength of the welded joint are 349 MPa and 440 MPa, respectively, the average elongation is 29. 3%. The bending specimens at different positions are not cracked at the bending angle of 90°. In addition to the samples containing pores, the impact absorption energy at room temperature is higher than 100 J, and it is higher than 65 J at low temperature. In general, the joints have good mechanical properties and meet the technical requirements.