4H-SiC is a promising next-generation semiconductor, though its high-quality surface is difficult to achieve at high efficiency. In this research, two kinds of polystyrene (PS)/CeO2 core/shell abrasives have been developed for formulating polishing slurry and employed in electrochemical mechanical polishing (ECMP) of 4H-SiC. The abrasives were synthesized by an in situ chemical precipitation process (PS/CeO2-A) and an electrostatic attraction process (PS/CeO2-B), respectively. The scanning electron microscope results showed PS/CeO2-B possesses more regular morphology than PS/CeO2-A. Their Young’s moduli (Ea) measured by an atomic force microscopy showed that PS/CeO2-A has a larger Ea than PS/CeO2-B, and it can increase by ∼68% when considering the abrasive bottom deformation. Subsequently, the polishing tests were conducted using PS/CeO2-A, PS/CeO2-B and CeO2, respectively. The results showed core/shell abrasives can achieve better polishing results than traditional CeO2 abrasive, and PS/CeO2-B is superior to PS/CeO2-A. Finally, the effect on MRR of shell thickness of core/shell abrasives was theoretically modelled, and this model was experimentally verified for the PS/CeO2-A abrasives. The results showed the highest MRR is achieved at an optimal shell thickness rather than the largest one, and the existence of the optimal thickness can be verified by the established model of MRR.