Liquid crystal is the core device of the control system of the spatial light modulator, and its phase modulation characteristic plays an important role in the drive design and characteristics analysis of the liquid crystal device. According to the theory of elasticity and dynamics equation of liquid crystal, the distribution of direction change of liquid crystal molecules with applied voltage is calculated by the difference method. According to the distribution and the electrically controlled birefringence of liquid crystal ,the relationship between the modulation phase of nematic liquid crystal and the driving voltage is obtained. Simulation results show that liquid crystal phase modulation can change from 0 to 4π, with the driving voltage varies from 0 to 5 V. Experiment results demonstrate that the 0~4π phase modulation change can be realized with the increase of electrode voltage, and the measurement of liquid crystal phase modulation are realized.