LN single crystal substrate plates subjected to various diffusion conditions are studied by XRD and SEM. The results show that there occurs a new phase structure in the Mg-rich layer on the diffusion surface, it is possibly some phase in Mg-Li-Nb-O system. This is thought to be the real source of the inward diffusion of Mg ions. Finally, the resulting distortion of LN lattice, the mechanism of inward diffusion of Mg, and the mechanism of reflectivity variation of the diffusion layer are analyzed and discussed.