The crosstalk of cascaded stimulated Brillouin scattering (SBS) in the S band distributed fiber Raman amplifier forward pumped by the fiber laser has been studied. By S band distributed fiber Raman amplifier using narrow spectral bandwidth (<100 MHz) as signal source, the forward stimulated Brillouin scattering was observed when amplified signal power is larger than the threshold of SBS, which is the phenomena that transmitting sound wave Brillouin scattering is amplified in fiber amplifier. Second-order SBS in the Stokes region appeared with the increase of pump power, and the power of even order SBS was stronger than that of odd order Brillouin-Rayleigh scattering in the experiment. The forward cascaded multiple stimulated Brillouin emerged when the pump power is increased further, and the amplified signal power is translated into SBS with stronger noise when the gain decreased. The crosstalk of SBS damaged the performance of Raman amplifier and made Raman amplifier useless in DWDM fiber transmission system, therefore the signal power and pump power entering fiber had to be controlled strictly. In the experiment, the signal and companion lines beside SBS could be observed, too.