Metal matrix composite parts fabricated by additive manufacturing are of interest due to their fine grain structures and superior mechanical properties. In this study, a phase-field (PF) model is developed to simulate the grain evolution of TiB₂/316L stainless steel composite during the selective laser melting (SLM) process. The reinforcing TiB₂ particles are explicitly incorporated into the PF model, and the influences on grain evolutions are comprehensively considered, including the induced heterogeneous nucleation and the pinning effect on grain boundaries. The simulation results demonstrate that with the addition of TiB₂ particles, both columnar and equiaxed grains are formed in the scanning track. While the columnar grains are due to epitaxial growth from the molten pool boundary, the TiB₂ particles act as the nucleation sites inside the molten pool, resulting in the formation of equiaxed grains. The grain boundaries become complicated due to the pinning of grain boundaries by TiB₂ particles. As the content of TiB₂ particles increases, the grain size decreases and the equiaxed grains are closer to the fusion boundary of the scanning track, which are attributed to the increasing nucleation sites and the retarded grain growth during solidification. In addition, the grain coarsening in previous layers is restrained due to the Zener pinning effect. The simulated grain morphology and the predicted influence trend of the TiB₂ content on grain morphology are in good agreement with the experimental observations in the literature.