A method to measure modulation property of liquid crystal based on diffraction grating was proposed. This method utilizes liquid-crystal to establish the binary phase gratings with the phase of 0 and φ respectively, the relation between intensity of zero-order diffraction spots and φ can be deduced by Fourier transformation, and the relation between intensity of zero-order diffraction spots and voltage of liquid crystal pole was measured by experiment. Therefore,relation between phase φ and voltage of pole liquid crystal can be achieved, that is the modulation property of this liquid crystal. Finally, the modulation property was used to creat liquid crystal phased array to steer laser beam, experimental results indicate that phase error is less than 1×10-3 rad.