7.5 ~ 9.7μm traditional wide band antireflective (AR) film is designed on ZnS, choosing Ge, ZnS and YF3 as film material, and the average transmittance (T) could be elevated from 74.9% to 98% when AOI=0°. But while AOI=70°, T of p and s component is 85.9% and 75.6% separately, and the average transmittance is 80.8% because of the separation of s- and p-polarized light. Depolarization antireflective film is also designed with the same materials by equivalent-layer method to balance s- and p- component energy, and the polarization effect at different incident angles is analyzed. When AOI=0°, T is 99.3%, and when AOI=70°, T is 90.6%, 10% less than that of traditional AR film. So equivalent-layer method is effective to eliminate polarization effect of broadband antireflective coatings at large incident angle and then could effectively improve the optical performance, which is critical in precision guide system.