Abstract: Welding tests were carried out on AZ31B magnesium alloy of two thicknesses（3mm and 6mm） using the CMT welding method, and the obtained welds were tested and analyzed in terms of appearance and morphology,microstructure, micro hardness, fracture and fatigue properties. The results show that: reasonable welding parameters can get optimized weld seam, and the metallographic organization is independent of the plate thickness. 3mm thick AZ31B magnesium alloy can reach 83% of the tensile strength of the base material, and 6mm thick AZ31B magnesium alloy can reach 88.7% of the tensile strength of the base material, which indicates that the brittleness of AZ31B magnesium alloy CMT welded joints increases with the thickness of the plate, and at the same time,the micro hardness of joints is overall in the form of W-shape. At the same time, the overall micro hardness of the joint is W-shaped distribution, the micro hardness of the weld core area is higher than the heat-affected zone and the hardness of the heat-affected zone of the thin plate is slightly higher than the thick plate. The fatigue strength of the joint under 2×106 cycles is 16.72MPa, which is only 7.8% of its tensile strength, and the fatigue strength of the welded joint is much lower than their tensile strength. The tensile fracture location of the joint is located in the base material, cut off at an angle of 45°, the fracture location coincides with the lowest microhardness location, the fracture region of the joint has obvious traces of plastic deformation.