Abstract: 2.5D SiC_f/SiC ceramic matrix composites exhibit characteristics such as high hardness, high brittleness, and heterogeneity, making them typical difficult-to-machine materials. To reveal the influence mechanism of planetary double-sided grinding process on the surface quality of 2.5D SiC_f/SiC ceramic matrix composites and achieve precise control of surface quality, a series of grinding experiments were conducted to quantitatively analyze the influence of sandpaper grit size on material removal rate (MRR), surface roughness (Sa), damage uniformity, and upper-lower surface consistency. The results showed that as the sandpaper grit size increases from 360 to 3 000, the material removal rate decreases significantly; when the grit size reaches 3 000, the abrasive action is mainly focused on dressing, and there is no obvious macroscopic material removal. The material removal rate does not change significantly after processing with sandpaper of higher grit sizes, indicating that the removal capacity of 3 000-grit sandpaper has tended to saturation. The characterization results of thickness and surface roughness also confirm that the regulatory effect of 3 000-grit sandpaper on surface quality tends to saturation. In terms of surface damage evolution: the surface ground by 360-grit sandpaper shows a large number of traces of fiber breakage and spalling as well as matrix brittle fracture. The characteristics of matrix brittle removal in the surface ground by 1 200-grit sandpaper are significantly reduced, and the fiber fracture morphology becomes more regular. 3 000-grit and 4 000-grit sandpapers further inhibit damage, and the surface is dominated by "plastic flow/mild brittle spalling", but there is no significant difference in the damage inhibition effect between the two. This study can provide a reference for efficient double-sided grinding and process optimization of 2.5D SiC_f/SiC ceramic matrix composites.