A grating-type infrared absorber based on LiF and NaF material composite grooves is designed. The finite-difference frequency-domain (FDFD) method is used to analyze the absorption mechanism. In addition, the influence of the structure parameters of the absorber, the wavelength of the incident wave and the incident angle on the absorption characteristics are investigated. The results show that this absorber will form different optical resonators for incident light of different wavelengths in infrared region, which can strongly absorb infrared light over a wide wavelength range. The absorber has a good absorption performance in the range of 18~70 μm wavelength range and 0~80° of incident by using the optimized parameters. The research work of this paper provides a theoretical basis for the design, fabrication and application of the grating-type infrared absorber.