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Prolonged polishing deteriorates the shape accuracy of an optical element and reduces production efficiency simultaneously. In order to reduce the amount of polishing and polishing time, even obtain polish-free fine surfaces, a cerium oxide (CeO2) slurry-enhanced grinding (SEG) is investigated. However, the lack of in-depth mechanism explanation limits the optimization and application of SEG. In this research, a novel theoretical model was established to predict the surface roughness of the workpiece processed by CeO2 SEG. The modeling considered the effects of the protrusion height of active grains in the grinding wheel and the sizes and mass fractions of CeO2 particles in the grinding zone on undeformed chip thickness (UCT). Then, the mechanism of CeO2 SEG was investigated through the nanoindentation method. Indentation hardness and energy spectrum of the surface were estimated to verify the softened layer. The results showed that the model of surface roughness was well consistent with the experiment. CeO2 particle size significantly influenced the surface roughness than the mass fraction. The load-bearing effect of larger CeO2 particle size reduced the protruding height of the grinding wheel grains and reduced the UCT to a greater extent in the grinding process. The chemical reaction between CeO2 slurry and BK7 glass results in a softening layer which enhances the critical load and critical depth of the ductile–brittle transition of grinding. Finally, the optimized parameters were used for CeO2 SEG of ellipsoid BK7 optics and obtained the high surface quality. [ABSTRACT FROM AUTHOR]