The stimulated Brillouin and Raman scatterings of circularly polarized pump in fluid plasma composed of electrons, positive and negative ions are investigated. Owing to the presence of negative ions, frequency and Landau damping rate of electrostatic idlers are significantly changed; i.e., the frequency and the Landau damping rate of the dispersion-free ion acoustic wave are raised by negative ions, while for the election plasma wave the frequency is lowered but the Landau damping rate is raised. And the relative decrease in electron concentration weakens the ponderomotive force. As a result, thresholds and growth rates of stimulated Brillouin and Raman scatterings are significantly changed by negative ions, all meaning that the initial stage of stimulated scattering process is weakened by negative ions when the pump intensity is far above threshold. However, when the pump intensity is close to threshold, the stimulated scattering may rather be enhanced by negative ions in cases where damping rate of the scattered signal is fairly greater than frequency of the ion mode(for Brillouin scattering) or damping rate of the electron mode(for Raman scattering).
