摘要： Circular beam oscillation was introduced to laser welding of 2 mm-thick AZ31 magnesium alloy. The appearance, microstructure and tensile properties of the welds were investigated. It was found that the low frequency about 50 Hz and the small radius about 0.5 mm are more beneficial to improve the weld appearance, under the given parameters of laser power 2 kW, welding speed 2 m/min and beam oscillating diameter 0.35 mm. The undercut defect and even the totally collapse of the weld appear when employing the frequency higher than 75 Hz or the radius larger than 1.5 mm. The microstructure evolution mainly reflects in the variation of the proportions of the equiaxed zone (PEZ) and the average grain size (SG). The PEZ decreases from 85% to 42% with the beam oscillating frequency increasing from 25 Hz to 100 Hz, while the SG reaches the maximum of 37.5 μm at 75 Hz. The PEZ decreases and the SG increases with the increase of beam oscillating radius. The results showed that the tensile strength and elongation are closely related to the PEZ, the SG and the twins. According to the experimental results, the relationship of oscillating parameters, microstructure, and tensile properties was established.