In order to increase the processability and process window of the selective laser melting (SLM)-fabricated Al-Mn-Mg-Er-Zr alloy, a novel Si-modified Al-Mn-Mg-Er-Zr alloy was designed. The effect of Si alloying on the surface quality, processability, microstructure, and mechanical properties of the SLM-fabricated alloy was studied. The results showed that introducing Si into the Al-Mn-Mg-Er-Zr alloy prevented balling and keyhole formation, refined the grain size, and reduced the solidification temperature, which eliminated cracks and increased the processability and process window of the alloy. The maximum relative density of the SLM-fabricated Si/Al-Mn-Mg-Er-Zr alloy reached 99.6%. The yield strength and ultimate tensile strength of the alloy were (371±7) MPa and (518±6) MPa, respectively. These values were higher than those of the SLM-fabricated Al-Mn-Mg-Er-Zr and other Sc-free Al-Mg-based alloys.