In this study, we investigated the influence of process parameters on the relative density of MS1 alloy manufactured by selective laser melting (SLM). The effect of post-heat treatments with 850 ℃×1 h solid solution + 550 ℃×4 h aging process on the microstructure and mechanical properties was elucidated. The SLMed MS1 alloy with high density and high performance was achieved. The results show that the structure of the SLM as-built alloy is mainly composed of lath marensite and austenite, with small size and uniform distribution, but there are tiny shrinkage holes in the as-bulit alloy. The post-heat treatments improve the as-built structure of MS1 alloy and eliminate pore defects. After heat treatment, much strengthening phases are precipitated in MS1 alloy, which increases its strength and reduces the ductility. The yield strength is increased by more than 1 time, the tensile strength is increased by more than 70%, the hardness is increased by more than 82%, and the elongation rate is reduced from 10.7% to 5.6%.