The nonlinear dielectric relaxation spectra of ferroelectric liquid crystals (FLCs) have been studied in the chiral smectic-C phase. The linear and third-order nonlinear dielectric spectra show the relaxation corresponding to the fluctuation in the azimuthal angle of directors called the Goldstone mode. We calculated the nonlinear dielectric spectra of the Goldstone mode theoretically by the torque balance equation which describes the dynamics of FLCs under the electric field. The calculated spectra make good agreement with the measured ones. We also evaluated the material constants of FLCs from the best-fitted values of the linear and nonlinear dielectric increment and relaxation time.