Abstract: Grating pair is the most important component of increment encoder, its structure and diffraction effcet are directly related to detection precision. The paper deeply analysed that the characteristics of diffraction single-grating and double-grating, and from the angle of spatial modulation deduce relation, grating pair influence Moire fringe. It analyzed the window function of the grating pair from both macro and micro perspectives, the uncertain reasons of the Moire fringe optoelectric signal waveform and contrast, and then proved that single-grating angle detecting can not only decrease frequency ingredient of output signal but achieve higher division precision and contrast. In order to prove the feasibility of single-grating angle detecting, this paper finally given a complete single-grating angle detecting way.