Abstract: In order to explore the effect of heat treatment on the mechanical properties of 18Ni300 maraging steel formed by selective laser melting(SLM),18Ni300 maraging steel samples were prepared by selective laser melting technology,and the formed samples were respectively treated by aging,solid solution,and solid solution+aging.The microstructure,microhardness,and tensile properties of SLM 18Ni300 martensitic aged steel were tested and analyzed by metallographic microscope,scanning electron microscope,hardness tester,and tensile testing machine.After solid solution+aging treatment,the laser melting pool of SLM 18Ni300 martensitic aging steel disappeared,and the structure was uniform and dense lath martensite,with fine precipitates dispersing in grain boundary and lath;the tensile strength and hardness increased significantly,but the elongation decreased significantly.In addition,the strong plastic deposition of the material can be improved after the solution+aging treatment.