Considering a two-level atom interacting with a single-mode thermal field, the reduced entropy change of the atom and the field with the sinusoidal and rectangular frequency modulations is studied by using quantum reduced entropy theory and numerical computation method. Results show that, for the sinusoidal modulation, the evolution of the entropy increments of the atom and the field presents periodic collapse and revival phenomena with large amplitude of the frequency change. For the rectangular frequency modulation, the intensity of entropy exchange between the atom and the field weakens significantly when the pulse strength increases. For both of the modulations, the entropy exchange between the atom and the field always occurs.