A novel spatial light modulator called reflector moving grating light modulator is proposed. Each pixel mostly consists of a fixed grating and a movable reflector. The optical performance of this modulator is a key parameter,so it is necessary to analyze the optical performance. With scalar diffraction theory,its optical performance is analyzed in details. Meanwhile,computer simulation is carried on,and the influence of several key parameters on the diffraction efficiency and contrast ratio is presented. All these results indicate that the height between the fixed grating and movable reflector must be controlled at λ₀ in off state,5λ₀/4 in on state respectively,and the occupation ratio of the fixed grating has an important influence on the optical performance whose optimized value is 0.5.The contrast ratio at different wavelength width is given out,wider wavelength,worse contrast ratio.