Diffractive surface of a two-layer diffractive optical element (DOE) is simulated approximately by step surface of binary optical element, and phase shift of a two-layer DOE for light of oblique incidence is deduced from phase shift of a single-layer binary optical element. The expression of diffraction efficiency of the two-layer DOE is presented. Simulation shows that the diffraction efficiency of a two-layer DOE changes slightly when the incident angle is small, but it decreases greatly when incident angle increases. Diffraction efficiency changes little when the incident angle increases to 4.5°, decreases slowly to 95% when the incident angle increases 6.7°, decreases to 80% when the incident angle change increases to 9.5°, and decreases rapidly to zero when the incident angle increases to 18°.