The Fluorine-containing silicon oxide film was deposited on a silicon wafer by plasma enhanced chemical vapor deposition. The optical parameters of the coatings were measured by ellipsometer and the effect of C₂F₆gas flow rate on the refractive index , the extinction coefficient and the deposition rate of the coating was studied when the pumping speed was fixed or the degree of vacuum is fixed. Experiments show that at 550 nm the refractive index is 1 . 37 and the extinction coefficient is 4 X10^-4in the case of temperature 300 益 , RF power 200 W, presure 20 Pa, SiH₄60 sccm, N₂O 40 sccm,C2 F6 30 sccm. The low-index coating can be used as optical antireflection coating materials.