Magnesium alloys have great potential in the field of biomedical degradable materials. However, their poor mechanical and corrosion resistance properties limit their applications. In this study, the AZ31B alloy with a compact hexagonal close-packed (HCP) crystal structure was selected as the material, as it tends to undergo twinning deformation rather than dislocation slip during plastic deformation due to its low stacking fault energy. The microstructures before and after LSP treatment were characterized using optical microscopy (OM), electron backscatter diffraction (EBSD), and X-ray diffraction (XRD). The results showed that the grain size was refined and exhibited a gradient distribution after LSP treatment. Electron backscatter diffraction (EBSD) was used to characterize the microstructure of LSP before and after treatment. The results showed that the grain size was refined after LSP treatment, and showed a gradient distribution trend.