This paper studied the effect of dopant-ion charges on the performance of thin film ferroelectrics. Field distributions and capacitance of ferroelectric thin films with different dopant-ion charge densities are analyzed. The non-linearity of the electric field inside the thin film increases with the dopant-ion charge density and decreases with the linear permittivity of the ferroelectric thin film. Once the applied voltage is strong enough, the field distribution will become linear. The FE thin film peak capacitance reduces with the dopant-ion charge density and the total area under the C-V curve between two large voltages with opposite signs is fixed.