The principle of designing an electric field controlled 2×2 bypass exchange switch in the photorefractive LiNbO 3 crystal is discussed. By controlling the magnitude of the applied electric field, the readout beam can satisfy or deviate the Bragg condition, which makes the diffraction efficiency be 1 or 0, and thus realize the exchange or bypass operation of the switch. Fully regarded the piezoelectric effect, the electrooptic effect caused by the applied electric field and based on the equation of diffraction efficiency and Bragg condition, the of between the applied electric field, the writing beam ratio with respect to the writing angles are analyzed.