With the rapid development of metal additive manufacturing technology, it has attracted people’s interest due to its high utilization of materials, complex structure and function of integrated design and manufacturing. It has been one of the most cutting-edge and most promising technologies. Metal additive manufacturing is actually an investigation of the matching relationship between materials, processes (process parameters / post-treatment), microstructure and properties. In this work, gas-atomized spherical powders of 17-4PH stainless steel was used in selective laser melting (SLM) technology, which employs laser beam to melt the powder layer by layer. Several cubic samples of 17-4PH stainless steel were fabricated under different process parameters. Then a solution heat treatment (1313.15K for 70min) and subsequent aging (743.15K for 90min) were conducted. The phase constitution of SLM-processed and heat treatment was determined by X-ray diffraction (Bruker D8 Advanced). OM, SEM, EDX and EBSD were used to characterize and analyze the microstructure of 17-4PH samples. Due to the high cooling rates in the process of SLM, the microstructure mainly consist of martensite and little austenite phase. For SLM-processed samples, the microstructure has obvious directionality and inheritance and epitaxial characteristics. In the vicinity of the fusion line showing a huge lath structure, and in the center of the melt pool presents the fine equiaxed grain, similar to the welding. This phenomenon was attributed to the heat flux and the thermal gradient of SLM, which is the characteristics of metal additive manufacturing. However, the fusion line was eliminated after heat treatment. The lath microstructure is transformed into an equiaxed shape with the precipitated phase and the orientation of the microstructure is weakened.