Grating-type ultra-broadband infrared absorber based on LiF and NaF is designed, and its absorption characteristics are researched by using finite-difference frequency-domain method. Research results show that grating-type absorbers consist of LiF(or NaF) and dielectric material, and they all have wider absorption band. The absorption bands are located at different infrared wavebands. The two absorption bands can be connected by using LiF, NaF and dielectric material together in the absorber. In the range of incident wavelength of 15-45 μm and the range of incident angle of 0°-80°, the absorptivity of the absorber is more than 80% with the optimization of parameters, which realizes broadband absorption. The layer number of the composite layer has the biggest effect on the absorptivity, and the minimal effect is the thickness of the dielectric layer.