The M50 aerospace bearing steel ingots are prepared using vacuum induction melting process. The effects of Ce addition, Mg addition and Ce-Mg simultaneous addition on oxygen content, sulfur content and inclusion distribution characteristics are analyzed comparatively. The results indicate that Ce can significantly enhance the efficiency of deoxidation and desulfurization. Preferentially, Ce addition will lead to the formation of Ce2O2S inclusions in steel. As the oxygen content in the steel decreases, Ce also reacts with As and purify the molten steel. Meanwhile, Ce also reacts with refractory materials to increase the number density of inclusions in the steel. When the Ce content is 0.018 wt.%, the smallest size and number of inclusions can be obtained. In addition to deoxidation and desulfurization, Mg addition can also inhibit the reaction between Ce and magnesia-aluminum spinel refractories. Thus, it is concluded that Ce-Mg synergistic treatment can significantly decrease the number and size of inclusions in the steel. The oxygen content in the steel has been decreased to be 7.5 ppm.