Recently, the significant change of OH spectra of highly Mg-doped LiNbO3 during polarization reversal process have been found experimentally. However, there is no proper theory and model to explain the unusual phenomena. Based on the Nb-vacancy model, the OH absorption band for highly Zn and Mg doped LiNbO3 single crystals under coercive field were calculated. Theoretical investigation of OH absorption band under the forward and back coercive field are also investigated. Our result shows the Zn2+ and Mg2+ ions sit at Li lattice sites for highly doped LiNbO3 single crystals. The theoretical prediction of the disappearing and the reappearing of the OH absorption band after polarization reversal agrees well with the experimental result, and this indicates that Zn2+ and Mg2+ ions do not substitute the Nb-lattice sites.