It is very important to obtain the accurate optical constants of thin films for the design and the manufacture of high-quality optical coatings, especially for those optical characteristics very sensitive to the variation of refractive index. SiO2 as a common low refractive index material is difficult in accurately fitting the optical constants which are close to those of the substrates. In this paper, single layer SiO2 film is prepared by ion beam sputtering. Considering the error in measurement and the influence of the substrate refractive index, the refractive index of SiO2 is obtained by the transmission envelop and the reflection envelop methods. The accuracies of these two methods are compared by inverting the obtained refractive index. The analysis shows that the refractive index of SiO2 with 10-2 precision can be well obtained by directly using the residual reflection of substrate and films because of the few errors in actual measurement of residual reflection.