Abstract: The cleanliness level and nonmetallic inclusion distribution characteristics of M50 aerospace-bearing steel are key factors affecting its quality and service life. The simultaneous addition of Ce–Mg has been proposed in this paper as an innovation to improve cleanliness dramatically. Based on the thermodynamic calculation, the underlying functional mechanism has been revealed. Additionally, the effects of Ce, Mg, and Ce–Mg simultaneous additions on oxygen content, sulfur content, and inclusion distribution characteristics have been analyzed comparatively. The vacuum induction melting process was used to prepare the M50 aerospace-bearing steel ingots. The chemical compositions of experimental steels were acquired using inductively coupled plasma-atomic spectroscopy, Leco TC500 N₂/O₂ analyzer, CS-3000 carbon/sulfur analyzer, and SPECTROLAB M11 stationary metal analyzer. The statistical distribution characteristics of inclusions were obtained using the image processing software based on optical microscopy images. The composition and morphology of inclusions have been characterized using scanning electron microscopy equipped with energy dispersive spectroscopy. The results indicated that Ce could significantly enhance the efficiency of deoxidation and desulfurization. Preferentially, Ce addition would lead to the formation of Ce₂O₂S inclusions in the steel. As the oxygen content in liquid steel decreases, Ce could also react with As to form a compound, and this could further purify the molten steel since As has generally been recognized as a harmful element. Meanwhile, Ce would also react with the magnesia–aluminum spinel refractory and cause an increase in the number density of inclusions in the steel. Thus, in comparison to the Ce-treated steel with higher Ce content, the smallest size and number of inclusions have been obtained in the steel with a total Ce mass fraction of 0.018%. In addition to deoxidation and desulfurization, Mg addition could also inhibit the reaction between Ce and magnesia–aluminum spinel refractories. The thermodynamic calculation results demonstrated that the dissolved Ce in the molten steel could react with the magnesia–aluminum spinel refractory material, resulting in an increase in the concentration of O and Al in the molten steel, while this reaction could significantly be inhibited by the dissolved Mg in the molten steel. In summary, Ce–Mg synergistic treatment could significantly decrease the number and size of inclusions in the steel. Based on this novel technology, the ultraclean M50 aerospace-bearing steel with an oxygen mass fraction of 0.00075% has successfully been obtained. This work has opened a new insight into the deoxidation mechanism of Ce–Mg synergistic treatment and provided a novel method to further improve the cleanliness of molten steel during the vacuum induction melting process.