SiO₂ is a very important low refractive index film material that can operate in the UV to mid-IR regions. The SiO₂ films used were deposited on Si substrates by different deposition techniques. We designed and manufactured a high-temperature infrared spectrum measuring equipment connected with a Fourier transform infrared (FTIR) spectrometer; the measuring temperature could range from room-temperature to 600°C. The FTIR transmission spectra of SiO₂ films under different working temperatures were measured in the wavenumber region from 4000 to 400  cm^−  1. From the measured spectra, it can be seen that IBS-SiO₂ and IAD-SiO₂ films are more stable and operating temperature can reach 400°C. EB-SiO₂ film is worse and operating temperature is only 300°C. Complex dielectric functions of SiO₂ films under different temperature conditions were calculated from FTIR transmittance spectra, and the best fitted method was obtained for calculating optical constants of dielectric materials in the high temperature condition. The results show that the structure of the Si-O-Si network in IBS-SiO₂ films was the most stable structure. The experimental results were of great significance to practical application.