The grating light valve (GLV), and optical modulator based on microelectromechanical systems (MEMS), with the changable components mobile ribbons on its surface, can serve as a diffractive grating, and be used in projection and display fields. The optical performance of GLV is analyzed in details and simulated with interference and diffraction theory. All these indicate that when the GLV is in on state, the mobile ribbons should drop a quarter of the incident wavelength. When the reflectivity of the air gap is different, the effect of the duty ratio on the optical properties is different. The best matching value of duty ratio and reflectivity is presented.