Ultra-wideband spectroscope is an indispensable optical component in spectral analysis system, and its performance directly affects the measurement accuracy of Fourier infrared spectrometer. Based on the optical film theory, the Ge, ZnSe and YbF₃ were selected as coating materials. The film system design was completed combined with the actual spectrum curves of the substrate, through the spectral compensation design method, and combined with TFC software. The electron beam evaporation and ion beam assisted deposition technology was used to prepare the thin film. Through the optimization of the deposition process, the problem of poor spectrum caused by large film thickness control error during the preparation of the spectroscope was solved. The Spectrum Two Fourier infrared spectrometer was used to detect that the average spectral ratio of 2.5 μm~20 μm band was 52:45.5 at 45° incidence. Through the environmental test, the spectroscope has good stability.