In this paper, the effects of attenuation coefficient and effective gain length on output energy of stimulated Brillouin scattering (SBS) in water are investigated theoretically and experimentally. The experimental results indicate that the smaller the attenuation coefficient, the higher the output energy of SBS is. When the energy of incident laser is very high and the effective gain length is long enough, the SBS may obtain high enough energy thereby reach an extremely strong peak power due to the pulse compression; once it exceeds the threshold of SRS or second-order SBS, the SBS is able to excite an SRS or a second-order SBS as a new source and consumes a part of its own energy. Therefore, the longer the effective gain length, the lower the output energy of SBS is.