In backward pumped fiber Raman amplifier (FRA), cascaded stimulated Brillouin scattering (SBS) is observed. In the experiment, a three-wavelength Raman fiber laser whose maximum output is 3000 mW was used as Raman pump, and 25 km single mode fiber (SMF) is used as both Brillouin and Raman gain media, an external cavity laser (ECL) whose power is 3 dBm is used as Brillouin pump. The backward cascaded SBS of 89 orders and forward cascaded SBS of 156 orders are observed. The frequency shifts of the backward and forward SBS between adjacent order are 22 GHz and 11 GHz respectively, namely their wavelength intervals are 0.166 nm and 0.085 nm respectively. The spectral bandwidths of the backward and forward SBS are 14.812 nm and 13.580 nm respectively. The flatness of backward spectral lines is better than that of forward. Compared with single-wavelength FRA, the three-wavelength Raman fiber laser used as Raman pump is easier to generate cascaded SBS.