Abstract: In order to improve the diffraction efficiency of read out light,reasonable parameters must be selected. Combining Kogelnik coupling wave theory with the Kukhtarev equations, the expression of the diffraction efficiency of the transmission grating in paraelectric KTN crystal was obtained. After analyzing the factors affecting the diffraction efficiency of a grating,it was found that the reading diffraction efficiency was sensitive to incident angle, crystal thickness and applied field. A high diffraction efficiency district existed in close vicinity of 45° incident angle. The influence of absorption of crystal on reading diffraction efficiency was negligible. The results provide useful guides to choose the optimal parameters for high reading diffraction efficiency in application.