Abstract: To study the influence of modulation error of electrooptic material on the quality of the interference fringes, an imaging model for interference fringes in electrooptic crystal was established. The relationship among the refractive index, the flatness and the wavefront modulation was analyzed, as well as the fringes imaging characteristics under the nonideal conditions. The experimental results show that the distortion of the refractive index of the crystal can causes the distortion of the interference fringes, the high harmonic components of the projection fringes are directly caused by the discrete order of refractive index, even severe distortion of projected fringes; In the case of the refractive index of linear growth, the crystal plane in a range of 0.1 μm can also cause interference fringes distortion.