Abstract: 10 mm thick AZ31B magnesium alloy was welded by the electron beam welding.The appearance, the macrophotograph, the microstructure, the alloy elements profile, the patterns and etc of welded joint were analyzed by means of OM, SEM, X-ray diffraction and etc.The results show that the appearance in the face of weld is perfect and the appearance in the back of weld has many tiny cupped hole.The ratio of the depth of fusion to the weld width is 8:1.The proportion of Al and Mn element has increased, and Mg, Zn has decreased in the welding zone.There are Mg, Al and a little Mg₁₇Al₁₂ in the welding zonewhere there are columnar grains of 8-18 μm.Hardness of the welding joint is almost uniformity.The strenghth of welding joint is 223 MPa.