In order to address the problem of poor surface corrosion resistance of Mg97Zn1Y2 alloy, a laser surface modification method that had emerged in recent years was adopted, and ultrasonic assistance was applied on the basis of laser surface modification treatment. The effects and mechanisms of ultrasonic-assisted laser surface melting (LSM) on the surface corrosion resistance of Mg-Zn-Y alloy were studied by observing the surface microstructure, elemental distribution, and corrosion behavior of samples with and without ultrasonic assistance. The results show that, although electrochemical test result indicates that ultrasonic assistance during LSM does not significantly improve the sample[DK]’s corrosion resistance due to surface roughness effects, the immersion corrosion test reveals that when the corrosion time exceeds 8 hours, the corrosion rates of the S160-15, S160-20, and S160-25 group samples are apparently lower than that of the S160-0 group; The thicknesses of the molten layers in ultrasonic-assisted laser-melted samples are positively correlated with ultrasonic amplitude, and all of them are higher than those of the experimental samples without ultrasonic-assisted laser melting; Electron backscatter diffraction analysis of the S160-20 group samples (with the best corrosion resistance) shows a delamination phenomenon in the molten layer, with 24.5 μm for the upper layer and 59.3 μm for the underlying layer. The corrosion behavior of the upper molten layer in the S160-20 group samples is consistent with that in the S160-0 group samples. However, grain refinement occurs in the lower molten layer of the S160-20 group samples, and the galvanic corrosion rate formed by these refined grains and long-period stacked ordered[DK](LPSO) phases is lower than that formed by the matrix grains and LPSO phases. Thus, the increased thickness of the molten layer induced by ultrasonic assistance is more conducive to improving the long-term corrosion resistance of Mg-Zn-Y alloy.