Abstract: In this study, 316H stainless steel was fabricated using selective laser melting (SLM). The evolution of its microstructure during the additive manufacturing process was systematically investigated. The results indicate that the SLM-fabricated 316H stainless steel exhibits a multiscale heterogeneous microstructure with anisotropic characteristics. On the printing plane (X-Y plane), the microstructure displays interlaced melt pool bands, while on the building plane (X-Z plane), it shows epitaxially grown columnar grains along the building direction and overlapping melt pools with a fish-scale morphology. Due to repeated thermal cycles, significant grain coarsening occurs along the building direction. Under the combined effects of low stacking fault energy and high cooling rates, the as-built microstructure consists solely of austenite, with no precipitation of other phases. The fraction of low-angle grain boundaries within the material is approximately 50%, accompanied by a high dislocation density. Notably, the dislocation density on the printing plane is significantly higher than that on the building plane, which is closely related to the thermal gradient and competitive grain growth. The study reveals that three mechanisms operate during the SLM process: competitive growth, plastic shear deformation, and dynamic recrystallization, with competitive growth being the dominant one. Texture analysis further confirms the presence of a growth texture (<110>//Z0, <110>//X0, <100>//X0), a deformation texture (<111>//X0, <112>//X0), and a recrystallization texture (<113>//X0). This work provides an important basis for understanding the microstructural formation mechanisms and tailoring the properties of additively manufactured 316H stainless steel.