Abstract: The anomalous diffraction patterns of 532 nm strong laser beam in LiNbO₃: Li; LiNbO₃: Li, Zn; LiNbO₃: Mg; LiNbO₃ single crystal are observed for the first time. The transmission curve and absorption curve of LiNbO₃ are measured. In addition, the anomalous diffraction patterns will not appear when the wave length of the strong laser beam is 694 nm. For the above experiment we propose an analysis of the mechanism of photo-induced fractive index charge of LiNbO₃.