A universal simulation method for 3- dimensional electro- optic device is proposed. This method combines the electro-optic wave coupling theory with finite-element method for electric field analysis. By solving Jones- matrix of electro- optic device, electro- optic modulation for arbitrary light propagating direction and arbitrary electric field direction can be computed. This method is elaborated by taking the transverse-modulated Bi₄Ge₃O₁₂(BGO) optical voltage sensor(OVS) for instance. The effect of inhomogeneous electric field and light beam shift on measurement accuracy is discussed. The maximum permitted beam shift for different accuracy grade is given. The method provides references for designing and evaluating optical voltage sensor and other electro-optic devices.