Stimulated Raman scattering can occur near the quarter critical density in direct-drive fusions, and the frequency of Raman-scattered light is about half of the incident light frequency. The second harmonic of the Raman-scattered light can be produced due to the inhomogeneity density profile. It can serve as the seed of stimulated Brillouin scattering (SBS). When the second harmonic of stimulated Raman-scattered light propagates against the incident light, some components will match the frequency of backward SBS, and SBS is induced. Thus, SBS could be greatly enhanced.