The influence of trace Y on the microstructure evolution and mechanical properties of Mg_100-xY_x (x=0.25, 0.75, 1.5, 3, 4, 5, at.%) alloys during solidification process was investigated via molecular dynamics (MD) simulations. The results show that the Mg_100-xY_x alloys are mainly characterized by a face-centered cubic (FCC) crystal structure; this is different from pure metal Mg, which exhibits a hexagonal close packed (HCP) structure at room temperature. Among these alloys, Mg_99.25Y_0.75 has a larger proportion of FCC cluster structures, with the highest fraction reaching 56.65%. As the content of the Y increases up to 5 at.% (Mg₉₅Y₅ alloy), the amount of amorphous structures increases. The mechanical properties of the Mg_100-xY_x alloys are closely related to their microstructures. The Mg_99.25Y_0.75 and Mg₉₇Y₃ alloys exhibit the highest yield strengths of 1.86 and 1.90 GPa, respectively. The deformation mechanism of the Mg-Y alloys is described at the atomic level, and it is found that a difference in the FCC proportion caused by different Y contents leads to distinct deformation mechanisms.