Abstract: The oscillating laser-arc hybrid additive manufacturing of AZ31 magnesium alloy was carried out, and the effect of oscillating amplitude on porosity, grain size, characteristics of grain boundary, and tensile properties of deposited components was investigated. The results have shown that the oscillating laser beam can effectively eliminate pores by stirring the molten pool and changing the flow state of the molten pool. The pores are completely eliminated as the oscillating amplitude reaches up to 0.8 mm. In addition, the oscillating laser beam can break the dendrites to provide massive nucleation cores, and it is beneficial to refine grains and promote the formation of twin boundaries. With the increase of the oscillating amplitude, the grain size decreases first and then increases, while the fraction of the twin boundaries increases first and then decreases. The grain size and the fraction of twin boundaries reach the minimum value of 23.2 μm and the maximum value of 35.2%, respectively, at the oscillating amplitude of 0.8 mm, representing a refinement and increase of 59.44% and 37.5%, respectively, compared with those without oscillating. The deposited component exhibits the best tensile properties at an oscillating amplitude of 0.8 mm. The tensile strength, yield strength, and percentage elongation after fracture along the horizontal direction reach 239 MPa, 102 MPa, and 25.8%, respectively, which are 27.8%, 27.5%, and 1.632 times higher than those without oscillating.