A theory of a nonlinear pyroelectric effect in a phase transition region of a ferroelectric thin film due to harmonic temperature changes with large amplitudes reaching tens of K has been developed. Polarization during the temperature cycle–pyroelectric hysteresis loop is obtained at various frequencies, amplitudes, thermal, and electric relaxation times. It is proved that ferroelectric film can be maintained in a monodomain state during the cycle. Applicability of a nonlinear pyroelectric cycle to solar energy conversion (ferroelectric solar cell) is discussed.