<p>The modification of sulfides in sulfur-bearing free-cutting steel can promote intracrystalline nucleation and reduce grain boundary segregation, thereby improving the chip formation and mechanical properties of the steel. Rare earth elements tend to modify MnS into complex inclusions; however, systematic and in-depth researches on this phenomenon remain limited. In current research, a high-temperature experiment was conducted on Y45Mn free-cutting steel to investigate the evolution behavior of inclusions during the modification process. First-principles calculations were combined with material characterization techniques to predict the crystal structure and growth characteristics of (Ce, Mn)S inclusions. Measurements and indexing of Kikuchi patterns revealed that (Ce, Mn)S inclusions share the same crystal structure type and similar lattice parameters with MnS and CeS. Subsequently, the atomic substitution model within MnS was constructed based on first-principles calculations. The results indicate that substitution of Mn atoms by Ce atoms is energetically favorable and introduces relatively small lattice distortion. Based on these findings, atomic adsorption and stacking growth models of (Ce, Mn)S on CeS surface were established on the low-index (100) crystal plane. The charge distribution during the growth process was calculated and analyzed to deduce the bonding mechanisms between atoms. The current research on (Ce, Mn)S solid solution inclusions provides valuable insights for the investigation of solid solution inclusions containing other elements in steel.</p>