Avalanche ionization model is introduced to the coupled stimulated Brillouin scattering (SBS) equations. Numerical simulation of SBS energy reflectivity as a function of pump intensity is made with and without the optical breakdown factor in the equations. In the experiment, purified and unpurified CCl４ media are studied with an Nd:YAG laser system with Q switch, the results are found to agree well with the numerical solutions. It is observed that under conditions that the optical breakdown does not occur, the energy reflectivity increases nonlinearly with pump intensity until reaching a certain value above which it tends to grow slowly. In cases with the breakdown, the energy reflectivity first increases nonlinearly with the pump intensity to a peak value and then decreases dramatically as optical breakdown occurs. The stability of energy reflectivity in the cases without optical breakdown is obviously better than cases otherwise.