The effects of the nonlinear polarization in a partially ionized plasma on the parametric processes of a strong laser are discussed. The nonlinear mode coupling equations and the linear growth rate of stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS) are derived. The numerical analyses for the nonlinear evolutions of SRS and SBS processes are given. Various effects of the second and the third order nonlinear susceptibilities on the SRS and SBS processes are discussed. The nonlinear evolutions of SRS and SBS processes are affected more efficiently than their linear growth rates by the nonlinear susceptibility.