The property of stimulated Brillouin scattering (SBS) excited by monochromatic light in absorption media is investigated. The distributed fluctuating source model is employed to describe the process of SBS built up from spontaneous Brillouin scattering. By using Fourier transform, we can obtain the power spectral density of the output Stokes wave, which is useful in predicting the threshold for the occurrence of SBS generation and linewidth of Stokes radiation.