Abstract: The phase conjugate laser oscillator based on stimulated brillouin scattering (SBS) can efficiently improvement the beam quality and realize a Q switch function. The mechanism of Q-switch based on SBS was analyzed, the result shows that the SBS effect belongs to fast Q-switch, which can be described by step function instead of consideration of the complicated Q-value changing process. The influencing factors on Q-switched characteristics were analyzed, including the back reflectivity, output coupler and SBS saturated reflectivity. The result shows that in designing of a SBS Q-switched laser, by ensuring the threshold to be reached, lower back reflectivity and higher SBS reflectivity are preferred and the output coupler reflectivity should be optimized also considering on the efficiency.