Normal incidence cannot fully reflect the working condition of diffractive optical elements (DOEs) for imaging. However, oblique incidence can reflect their general working condition. The relationship between the diffraction efficiency and the incident angle is established for triple-layer diffractive optical elements (TLDOEs). For the three influencing factors, such as the determination of incident direction, the selection of the substrate materials on both sides and the selection of medium materials, the relationship between the diffraction efficiency and the incident angle is simulated and analyzed for TLDOEs. The results show that the diffraction efficiency decreases slowly with the increase of incident angle when the light is incident from the positive harmonic diffractive element (HDE). The larger of the difference of abbe number for the substrate materials of TLDOEs is, the more slowly of the corresponding diffraction efficiency decreases with the increase of incident angle. The selection of the medium material is decided by the incidence angle range of TLDOEs. The analysis method and conclusions are applicable to the visible or infrared TLDOEs.