Abstract: AZ31 alloy has the characteristics of high specific strength, good electrical and thermal conductivity, and is widely used in transportation, electronics industry, aerospace and other fields. AZ31 magnesium alloy thin-walled components with fine equiaxed grains were successfully prepared by cold metal transfer（CMT）. The effects of spiral oscillation on the microstructure and mechanical properties of AZ31 magnesium alloy fabricated by arc additive manufacturing were studied. The results show that the arc oscillation can effectively refine the grain size of AZ31alloy. The average grain size of the samples without swing and arc swing is 46.9μm and 36.7μm. The ultimate tensile strength of the transverse tensile sample and the longitudinal tensile sample under arc oscillation reaches 216MPa and 221MPa, respectively, which is significantly better than that of the sample prepared without arc oscillation.