With the theory of one-dimensional approximation, combined with the boundary conditions, an inhomogeneous electric field is derived from the foundamental equations of pieozoelec-tric effect. This inhomogeneous electric field will lead to generation of "electric grating", and "high-order grating" can also appear owing to coupling between "electric grating" and "acoustic grating". Linear electro-optic effect in PESO modulator is helpful to the diffraction and modulation of the incident light beam. The experimental results verify our conclusions.