Abstract: The influence of laser power, scanning spacing and scanning speed on the relative density of selective laser melting formed 12Cr9NiAlMo steel was studied by orthogonal experiments, and the optimal process parameters were obtained. The structure and properties of samples formed by the optimal process after solution and aging treatment were studied. The results show that the laser power had the greatest effect on the relative density of the formed samples, followed by the scanning spacing. The optimal process parameters were laser power of 325 W, scanning space of 0.10 mm, and scanning speed of 1 000 mm·s^-1, and the relative density was 99.22%. After heat treatment, part of the residual austenite in samples formed by the optimal process was transformed to martensite, and a large number of fine NiAl phase precipitated. The tensile strength, yield strength and hardness increased by 62.1%, 59.6%, 41.2%, and the percentage elongation after fracture and impact absorbing energy decreased by 40.0%, 81.3%, comparing with those without heat-treatment.