SiO2 thin films is one of the most important low refractive index materials in the field of optical thin film. SiO2 thin films were deposited on Si substrates by different deposition technique. The optical stability of SiO2 thin films were investigated as a function of time placed in the air. Optical constants of SiO2 thin films were calculated using the ellipsometry spectra at different aging times. With the increase of the placed time, the physical thickness and optical thickness of EB-SiO2 thin films and IAD-SiO2 thin films increase, but IBS-SiO2 thin film decreases. The change of optical thickness of SiO2 thin films are separately 1.0%, 2.3% and -0.2%. When the placed time reaches 120 days, the physical thickness and optical thickness of IBS-SiO2 thin films, EB-SiO2 thin films, and IAD-SiO2 thin films tend to be stable. The results indicate that the optical stability of IBS-SiO2 thin film is better than other SiO2 thin films, and in the design of protective coating, SiO2 thin films should be deposited with ion beam sputtering technique.