According to phase shift method of binary optics, the phase delay expression and the diffraction efficiency of Harmonic Diffractive Optical Element (HDOE) at oblique incidence were derived. And the relationship between the diffraction efficiency and the beam incident angle was qualitatively analyzed. The results show that in the visible spectral range, resonant wavelength of HDOE becomes longer with incidence angle increasing, otherwise in the short-wave part wave crest drift more violently; with incident angle of monochromatic light increasing, the diffraction order sequence appears; with the incidence angle increasing, the wave crest of ordinary binary element drift much faster than HDOE; beam incidence angle has a greater impact on diffraction efficiency of short wave.