The present study is devoted to research the effect of process parameters on processability, and aging treatment on microstructure and mechanical properties of selective laser melting (SLM) high Mg-content Al-Si-Mg-Zr alloy. The results show that alloy forms a large number of fine equiaxed grains at the boundary of the molten pool, which can avoid the generation of cracks during the SLM forming process and increase the processability of the sample. The porosity of the samples obtained under different process conditions is less than 0.3%. The yield strength (YS) and ultimate tensile strength (UTS) of the as-built samples are 426 ± 8 MPa and 464 ± 12 MPa respectively. After aging treatment, due to the increase in the number of nanoparticles embedded in α-Al cells, the strength of the sample increases obviously. The maximum YS and UTS of aged samples reach 482 ± 11 MPa and 522 ± 10 MPa, respectively. The strength of present samples is much higher than that of commercial SLM-formed Al-Si-Mg alloy.