Selective laser melting (SLM) technology is used to fabricate S-04 specimens, and their microstructures and mechanical properties before and after heat-treatment are researched. The results show that the density of S-04 molded sample can be up to 99.791% and the specimens before and after heat-treatment are both composed of α′ martensite and a little residual γ austenite. The untreated specimen possess a multi-layer stacked character peculiar to the SLM technique, whereas after heat-treatment, this character disappears and the main specimen phase is bunchy martensite. The mechanical property at room temperature of heat-treated specimens is better than that of non-heat-treated ones.