Electrorheological (ER) finishing is a promising technique for polishing optical mirrors lenses. Silicone oil-based ER finishing fluid with ceria particles as abrasive was developed, whose dispersed phase was alumina, titanium dioxide, silica, and starch, respectively. Experiments were performed in detail under the voltage of 3000 V, after 25 min of polishing a K9 glass. By using the ER fluid developed in this paper, with 47.62% starch, 47.62% silicone oil, and 4.76% ceria, the surface roughness of the K9 glass was reduced from 8.46 nm Ra to 3.45 nm Ra. The result verifies the validity of the developed ER fluid.