The electro diffraction phenomenon of liquid crystal is studied.The experiment shows that when thedriving voltage is appropriate , the liquid crystal molecules will appear a periodic distribution structure similar tothe grating,that is ,the liquid crystal diffraction grating.The diffraction process and diffraction characteristics ofiquid crystal grating are explained by the superposition of light waves with different polarization states produced by two staggered superposition gratings ,and the distribution of diffraction pattern and the change lawof polarization state are studied.