The principle of selective optical amplification of weak scattering signal in water based on Brillouin amplification is presented. Based on the theoretical model of Brillouin amplification with frequency detuning, the influences of linewidth and frequency detuning of Stokes seed beam on Brillouin amplification are analyzed. The dependence of seed amplification factor on water temperature and seed power density are investigated. The theoretical calculations coincide with experimental results. Researches show that, taking the same frequency detuning into account, amplification factor of a spectrally broadened seed beam is higher than that of a narrow-linewidth signal beam. When 20℃ CS2 is used as amplifier medium, Stokes seed signals produced by different-temperature seawater are efficiently amplified. At the same pump power density, the amplification factor decreases with increasing signal power density. The amplification factor exceeds 102 for signal energy of 5μJ.