The influences of the delay time, the cell length and the pump energy on the gain amplification of weak Stokes signal of stimulated Brillouin scattering (SBS) in water are investigated theoretically and experimentally. In the experiment a frequency-doubled seed injection Nd:YAG laser with a pulse duration of 7.2 ns was used. Results showed that the pump pulse should lag the Stokes signal pulse by an amount equal to pulse half-width to obtain the maximum gain. Depending on the pump energy the signal below 10-5 mJ is linearly amplified. By using CS2 as amplifier medium, a Stokes signal of 1pJ from water is amplified with high gain (7×107) in the regime below the pump's self-SBS threshold.