Based on the expression of diffraction efficiency for multilayer diffractive optical element, the relationship between the air gap of a two-layer diffractive optical element and diffraction efficiency is derived. The influence on diffraction efficiency of the diffraction optical element in visible spectrum, infrared medium wave and infrared long wave from air gap, period and incident wave is analyzed. Taking 400~700 nm, 3~5 μm and 8~12 μm wavelength as examples, with polycarbonate (PC), poly methyl methacrylate (PMMA) as the first layer basement and the second layer basement of the two-layer diffraction optical element in visible wavelength respectively, with zinc sulfide (ZnS) and zinc selenide (ZnSe) as the first layer basement and the second layer basement of the two-layer diffraction optical element in infrared medium wave and infrared long wave. In particular period situation, the influence on diffraction efficiency of multilayer diffractive optical element from the size of air gap of two-layer diffraction optical element is determined.