Bi₂Ti₂O₇ thin films were deposited on n-GaAs(100) by chemical solution decomposition technique. The infrared optical properties of the thin films were measured by infrared spectroscopic ellipsometry in the wavelength range of 2.8～12.5μm. Lorentz-Drude dispersion model was used to express the infrared dielectric constants of the Bi₂Ti₂O₇ thin films, and the refractive index n, the extinction coefficient k and the absorption coefficient α of the thin films were calculated. The thickness of the Bi₂Ti₂O₇ thin films was obtained as 139.2 nm by fitting.