With the development of the semiconductor integrated circuit, flat panel display, and photovoltaic solar energy etc. Thin film materials with excellent optical properties in infrared band are widely used in many fields. If the film thickness of these infrared thin-film can be measured with high precision and speed, it is of great significance for improving performance of products. As a best means of film detection, the technology of spectral ellipsometry is with the features of non-contact, non-destructive, high-precision, and multi-parameter measurement. However, the existing spectroscopic ellipsometry cannot meet the requirements for the measurement of optical films in the infrared band. For this purpose, a technology of thin-film ellipsometry measurement based on Fourier infrared spectroscopy is proposed .According to the technical principle, an infrared ellipsometer with wavelength range of 2100~ 3200nm is established. The feasibility and accuracy of the ellipsometer are verified by testing standard SiO₂ - Si sample with different thicknesses. The results show that the measurement accuracy of thin-film thickness is±0.5nm in the range of 2100~3200nm, the thickness repeatability measurement accuracy is 0.07nm, and the single point measurement time is approximately 1min.