The influence of different amounts of cerium on inclusion evolution, solidification structure, elemental segregation, and intermetallic precipitation in super-austenitic stainless steel S31254 were systematically investigated via a combined experimental characterization and thermodynamic calculations. The research results indicate that the mass fraction of total oxygen (T.O.) in the proposed steels decreases from around 0.0137 to 0.0065 pct with the addition of Ce. The evolution trend of inclusions is Al2O3 → Al2O3 + CeAlO3 → CeAlO3 + Ce2O3 → Ce2O3 with the increasing Ce content. Systematic analysis of the size and area fraction of inclusions in the steel samples reveals that the dispersion effect of Ce on inclusions is most effective when the mass fraction of Ce is 0.0095 pct. With the addition of Ce, both the Ce-containing inclusions and the solute Ce contribute simultaneously to refine the solidification structure, effectively reducing the segregation rates of chromium, nickel, and molybdenum elements and weakening their microsegregation behavior. The addition of Ce also significantly promotes the formation of precipitated phases, such as σ phase, although the growth of these phases is restrained as the element segregation ability decreases. Moreover, this study comprehensively investigates the effect of rare earth alloying on inclusions, microsegregation behaviors of other key elements, and secondary-phase precipitation during the solidification process of S31254 steel. The current work sheds light on the material design of advanced stainless steels with hyper properties, as well as providing the roadmap for further alloying super-austenitic stainless steels by rare earth addition.