Electric field distribution of the light modulator affected by the dioxide dielectric charging is deduced with Gauss law. Electrostatic force formula of the movable grating under such condition is obtained. Charging and discharging mechanisms of the dielectric have been analyzed. The relationship between movable grating's displacement and the dielectric charge is obtained. The results show that the grating will bounce back and then be pulled down when it is actuated by low frequency voltage. When the actuating voltage period is far shorter than the charging time constant, variation of the modulator's output light intensity will be increasingly small until the modulator is disabled thoroughly when the field produced by the storage charges is half that of the actuating electric field. Experiment was carried out to test the theoretical analysis and the results agree well. Finally, a method is proposed to eliminate the storage charge, and its feasibility is proved by the experiment.