To achieve fast zooming by using electro-optic (EO) crystal, the EO crystal and electrode structures are required to be designed properly. Based on electro-optic effect, the basic principle and way of the EO crystal design were proposed. By means of simulation and comparison of internal non-uniform electric field and total additional optical path between linear EO crystal(lithium niobate crystal) and square EO crystal(tantalum potassium niobate crystal), the optimization of the EO crystal and electrode design were obtained. On this basis, the performance of the EO crystals for fast zooming was simulated and analyzed. The influence of the applied voltage and crystal thickness on the total additional optical path was discussed. Results show that the curvature radius of additional optical path decreases with the increasing of the control voltage, while increases with the increasing of the crystal thickness for a given applied voltage. In practical applications, the crystal thickness and applied voltage need to be considered comprehensively to achieve better zooming effect.