The nonlinear optical properties of the strongly coupled exciton-phonon systems are investigated theoretically without making any approximation for excitons. It is shown that the nonlinear optical absorptions and Kerr coefficients are enhanced significantly as the frequency detuning value of the signal field and the excition is equal to the phonon frequency, which makes clear that the strong excition-phonon interaction affects the nonlinear optical properties of the medium greatly. The numerical results and the detailed comparison also indicate that the nonlinear optical absorptions and Kerr coefficients are further magnified without making Bose approximation and the Dyson-Maleev approximation for excitons.