50~690 ppm cerium-containing steel samples were prepared in a 50 kg vacuum induction furnace. The morphology, composition, and size of rare-earth inclusions in steel with different cerium contents were characterized by SEM, EDS, and Feature software. The results show that with the increase of cerium content, the inclusions will undergo the transformation of MnS + Al2O3 → CeAlO3 + Ce2O2S → Ce2O2S + CexSy+ CaS → CexSy+ CaS → CexSy+ CaO → CexSy+ Ce2C3+ Ce2Si2O7 + CeP + CeAs. The number and size of class A (MnS) and class B (Al2O3) inclusions decrease statistically, and the number and the size of class D and class Ds inclusions decrease first and then increase. The density of inclusions decreased from 0.43 to 0.4 μm2, and then increased to 0.44 μm2.The appropriate amount of rare-earth addition can promote the uniform distribution of inclusions. Through high-temperature equilibrium calculation, it is concluded that CeAlO3, Ce2O3, Ce2O2S, CexSy, and CaS are produced in the smelting process. MnS, CeAs, CeP, CeN, and Ce2C3 inclusions do not form above the liquidus temperature. The inclusion transformation of O–S–Ce, Si–S–Ce, Si–O–Ce, N–O–Ce, N–S–Ce, and Al–O–Ce systems is calculated, respectively. The Ohnaka segregation model was used to calculate the formation process of MnS, CeAs, CeP, CeN, and Ce2C3, and it was found that these inclusions are easy to form at the end of solidification. The kinetic analysis of the size of rare-earth inclusions shows that the small critical size radius and small polymerization force of rare-earth inclusions are the main reasons for the small size. The mechanism of rare-earth-modified Al2O3 and MnS at 1600 °C and 1454 °C is calculated, respectively, and the rare-earth content of modified inclusions under different total oxygen content is obtained, which provides a theoretical basis for the application of rare-earth Ce in microalloyed steel.