Laser powder bed melting of magnesium alloys is mainly aimed at traditional industrial cast magnesium alloys such as AZ91D, ZK60, and WE43, which usually have lower tensile strength. On this basis, we developed a new high-strength Mg-12Gd-2Y-1Zn-0.5Mn (wt.%, GWZ1221M) alloy for LPBF additive manufacturing process, and systematically studied the microstructure and mechanical properties evolution from the construction state to the LPBF-t4 and LPBF-t6 states. As cast GWZ1221M alloy exhibits fine equiaxed grains with an average grain size of only 4.3 ± 2.2 μ m. As cast alloys exhibit typical coarse dendrites (178.2 ± 73.6) μ m) . The results show that the yield strength (YS) of the cast alloy is 315 ± 8 MPa, the ultimate tensile strength (UTS) is 340 ± 7 MPa, and the elongation (EL) is 2.7 ± 0.5%. Solid solution treatment makes it hard and brittle β- The transformation of (Mg, Zn) 3 (Gd, Y) phase into base phase X phase and layered long-period ordered stacking (LPSO) exhibits good plastic deformation ability, thereby improving elongation. Then, peak aging heat treatment introduced a large amount of nano prismatic shapes inside the grains β′ The precipitation resulted in the enhancement of YS. Finally, the YS, UTS, and EL of LPBF-T6 alloy were 320 ± 3 MPa, 395 ± 4 MPa, and 2.1 ± 0.4%, respectively, achieving high strength. The tensile strength of finished products and LPBF-T6 GWZ1221M alloy is significantly higher than that of as cast and finished Mg gd alloys, highlighting the enormous potential of high-strength finished Mg gd alloys in structural applications.