The theory model is transient model including the effect of the pump depletion. The results of numerical simulation are consistent with the experimental results. The picoseconds pulse was obtained by two-stages stimulated Brillouin Scattering (SBS) compressor to compress 8 ns laser pulse. The experiment used CCl4 as SBS medium. The laser pulse was compressed to 1.5 ns by the first stage compressor. In the second compress stage, by the use of 532 nm and 1064 nm as a pump, separately, the shortest pulse of 60 ps was obtained. The results show that the pulse with the duration shorter than the medium acoustic lifetime can be achieved by SBS compressor. The results of numerical simulation are consistent with the experimental results.