To investigate the forming quality of selective laser melting (SLM) in an aerobic environment, Sn-3.0Ag-0.5Cu (SAC305) alloy with a low melting point was selected. An orthogonal experiment scheme involving three process parameters (laser power, scanning speed, and hatch spacing) was designed to investigate the effects of different process parameters on the density of a sample formed via SLM in an air environment. Additionally, the surface morphology, micromorphology, and mechanical properties of samples with different densities were analyzed. The results indicate that laser power affected the density of the samples the most, whereas the other parameters exerted insignificant effects. The density of the samples increased with the energy density. For samples with lower densities, the main internal defects are holes and non-fusion defects. As the density increases, non-fusion defects disappear gradually and the number and size of holes decrease. The mechanical properties of the specimen formed in an aerobic environment are primarily affected by the residual large-sized spatters inside the sample. The failure of spatters to fuse with the substrate reduced the cross-section area of the sample and decreased its tensile strength. Compared with samples formed with protective gas, the abovementioned sample shows only a slight increase in oxygen content, although the oxidized spatters affect the forming quality.