In order to study the influence of different heat treatment processes on the microstructure and mechanical properties of 18Ni300 stainless steel manufactured by laser selective melting, Aging, solid solution + aging treatments were performed on the samples respectively. The phases, microstructure and mechanical properties of different heat-treated samples were tested by X-ray diffractometer, metallographic microscope, scanning electron microscope, tensile testing machine and hardness tester, etc.. The results show that the content of retained austenite in printed and aged samples is slightly higher than solid solution + aged state. Comparing with printed state, the laser melting pool boundary of aged sample becomes shallower, and the microstructure is mainly small lath-like horses. The boundary of the laser melting pool of the aged sample becomes shallower, and the structure is mainly fine lath martensite, and there are more fine Ni3Ti and other precipitates dispersed in the martensite boundary. The laser melting pool boundary of solid solution + aging state sample can no longer be distinguished, the microstructure is uniform and fine lath martensite, and the precipitated phases are distributed at grain boundary, martensite lath boundary, and dislocation interlacing; Aging and solid solution + aging treatment significantly improve the strength and hardness of samples, but reduce the toughness and plasticity. The printed tensile fracture has obvious necking, there are a large number of large dimples and a certain amount of holes in the fracture fiber area. There are obvious signs of tearing at the edge of dimples. The fracture form is ductile fracture. After aging, solution + aging treatment, it is mainly shallow dimples, and the small isometric dimples of shear lip are significantly smaller than printed state.