High and stable values of phase conjugation (PC) fidelity are available by use of a two-cell stimulated-Brillouin-scattering (SBS) PC mirror. Making SBS return to YAG Q-Switched oscillator and output coupled with polarizer, the SBS output pulses take an advantages of short, tunable and stable good quality. The principle of compressing pulses and the influence of PC mirror's relative positions on SBS pulse width are analysed. A SBS pulse with 2.5 ns width and 9 mJ energy is obtained.