Abstract: In order to study the electro-optical effect of LiNbO3 crystal under condition of non-paraxial when electric field was constant, the cyclic coordinate transformation equation of ellipsoid tensor of refractive index was adopted to deduce the expression of refractive index distribution in the optical axis orthogonal region. The dependence of refractive index, phase delay and transmittance upon the direction of incident laser was discussed. The experimental data of the transmittance changing with α, the angle between light beam and optical axis, were obtained. The results show that when the electric field is applied along y axis, a slight deflection of the incident light will lead to a significant change of the x-direction refractive index and beam transmittance whereas the y-direction refractive index has no change. The angle α between light beam and optical axis affects the electro-optical effect of LiNbO3 crystal far greater than the angle β between light vector in the x-y plane and the positive of x axis. The induced refractive index remains similar when adding electric field both in x axis and y axis but the former phase delay is smaller. The transmittance of the former also has a slower change than that of the latter between 0° and 0.4399°, Moreover, after the angle of 0.45°, the transmittance changes alternately at the positions of 0.5°,0.680°and so on, the transmittances in x axis and y axis are just the same. The result has a certain significance to improve the electro-optical properties of LiNbO3 crystal by means of angle adjustment.