In order to obtain the optimal scanning speed of laser surface modification, taking the AZ91D magnesium alloy with wider engineering application as the substrate, the organization of the laser modified alloy sample by different scanning speed was measured, the variation of modified layer structure with laser scanning speed was analyzed. The results show that: with the increase of the laser scanning speed, the content of β-Mg17Al12 in modified layer and the size of the dendrites reduce, and it has the distribution characteristics of directional solidification.