Dielectric dispersion of relaxor ferroelectric in diffusion region is related to the doping of extrinsic ions or ordering arrangement of intrinsic ions. The further explanation and unitive characterization of the dispersion property are required. Focusing on the facts that cation vacancies and dielectric dispersion in barium titanate based relaxor ferroelectric can be caused by donor doping in high concentration, three equivalently correlated Gaussian distributions of doping dopants, Curie temperature, and cation vacancy are obtained. The relevant Gibbs free energy and relation between dielectric constant and temperature of the relaxors with donor doping are deduced. The result demonstrates an increase in concentration of cation vacancy with temperature decreasing in diffusion transition region. The unipole charges caused by the cation vacancies are probably responsible for the dielectric dispersion and loss by their hopping motion.