Abstract: 12CrNi2 alloy steel was additively manufactured using selective laser melting (SLM) technology. The influence of laser energy density on the microstructure and mechanical properties of the SLM-formed alloy steel has been studied using methods such as metallographic microscope, scanning electron microscope, transmission electron microscope, microhardness tester, and room temperature tensile test. The results demonstrate that the macrostructure of the SLM-formed alloy steel can be divided into two parts: molten pool zone and heat affected zone. The microstructure consists of tempered martensite and a small amount of retained austenite. With the increase of laser energy density (E_V), the pore defects in the SLM-formed alloy steel are gradually reduced, and the density is gradually increased, which can reach 99.87%. In the meanwhile, the molten pool volume and lifetime increase and the cooling rate decreases, resulting in the widening of the tempered martensite lath and the heat-affected zone; further, the microhardness and strength of the alloy steel are decreased, and the plasticity is increased. When E_V is 81.34 J/mm³, the SLM-formed 12CrNi2 alloy steel exhibits optimal tensile properties, its tensile strength and yield strength are 1098 MPa and 882 MPa, respectively, and its elongation is 20.07%. The comprehensive mechanical properties of 12CrNi2 alloy steel formed by SLM technology are better than those formed by laser melting deposition (LMD) and casting technology.