In this paper, a digital orthogonal phase grating with adjustable period and phase modulation depth is simulated by using a programmable spatial light modulator. On the basis of analyzing the structure of orthogonal phase grating, its transmission function, far-field diffraction patterns, and diffraction efficiency are derived and calculated, and the influence of grating period T and phase modulation depth ? on the diffraction characteristics of orthogonal phase grating is discussed. The simulation and experimental results show that as ? increases from 0 to π, the relative intensity of zero-order light decreases gradually, and the intensity of eight first-order lights enhances. When ? equals π, the relative intensity of zero-order light reaches the minimum value of 0.25, and the maximum diffraction efficiency of first-order light is 56.96%. As ? increases from π to 2π, the relative intensity of zero-order light increases gradually, and the intensity of eight first-order lights decreases. This research provides a theoretical basis for the application of orthogonal phase grating.