In this paper the behavior of degenerate four wave mixing and parameteric oscillation of backward wave under strong pumping condition is analysed theoretically. Considering the competing effect of Raman scattering, the dependence of intensity of parametric backward wave oscillation on the pumping intensity is obtained. The theoretical result agrees well with the experiment for nitrobenzene. The analysis also shows that the intensity of parametric oscillation is a multi-valued function of the pumping intensity due to the intensity-dependent attenuation coefficient.