The off-axis three-mirror optical system has been widely adopted as the design scheme for spectral imaging system because of its wide service band, high image quality, controllable stray light and other advantages. In this paper, an optical system is designed using Code V. Based on the theory of polarization of light, the conversion relationship between film samples and the polarization fidelity and phase difference of the system is derived. After that, with aluminum substrate as the base material, and silver, alumina and titanium dioxide as coating materials, the reflective film is developed based on the basic theory of optical thin films, which features the incident angle of 38.5°, R_S>99.96% andR_P>99.8% in the range of 1 545-1 555 nm, and the phase delays of P-light and S-light smaller than 1°. Film design and simulation analysis are completed in the film design software. Using the high-performance optical coating machine designed by Leybold Optics, the aluminum substrate reflective film for the off-axis three-mirror optical system is prepared. S and P reflectance spectra and the phase of the coating samples are tested by Lamda1050 spectrometer. The results meet the design specifications. The research is of great significance in practice and engineering projects.