In　this　work,　Er-Zr-Sc　modified　Al-Mg　alloy,　manufactured　by　selective　laser　melting,　offers　excellent　mechanical　properties,　due　to　the　formation　of　a　desirable　microstructure　and　the　synergistic　effects　of　multiple　strengthening　mechanisms.　The　microstructure　in　as-built　and　aged　alloys　have　been　characterized　by　scanning　electron　microscopy　and　transmission　electron　microscopy.　The　results　show　that　the　microstructure　of　alloy　is　composed　of　equiaxed　grains　(-0.7　mu　m　in　diameter)　along　the　boundary　of　molten　pool　and　columnar　grains　(-1.46　mu　m　in　width)　inside　the　molten　pool.　Al3(Sc,Zr)　particles　serve　as　nucleation　sites　to　promote　the　nucleation　of　equiaxed　grains,　and　Al3(Er,M)　eutectic　phases　and　other　compounds　at　the　grain　boundaries　inhibit　grain　growth,　both　of　which　greatly　refine　the　grains　and　bring　significant　grain　boundary　strengthening　and　Orowan　strengthening　effect.　During　the　aging　process,　coherent　Al3(Er,Zr,Sc)　nano-precipitates　are　dispersedly　precipitated　from　the　Al　matrix,　which　plays　a　key　role　in　improving　the　mechanical　properties　of　the　aged　alloy,　and　then　the　alloy　reaches　a　yield　strength　of　530　MPa,　a　tensile　strength　of　up　to　542　MPa,　and　an　elongation　of　over　12　%.　Under　the　combined　addition　of　Er,　Sc,　and　Zr,　and　the　influence　of　multi-element　interaction,　the　alloy　has　high-density　secondary　phases　and　extremely　fine　grains,　which　are　the　main　sources　of　reinforcement.