Electrooptic effect of crystals is very important in modern physics experiment, which involves refractive index of crystals changed by extra electric field, so it is abstract and difficult for students.The phenomenon that index ellipsoid of crystals and interference can be changed by extra electric field is shown by computer simulation in this paper, and many important concepts such as halfwave voltage, direct current modulation and active current modulation is clearly explained, which plays a helping role in teaching and studying of this experiment.