The phase conjugation effect and the threshold effect of stimulated Brillouin scattering (SBS) on the divergence angles of back SBS were discussed in the theory. In the experiment, the active medium and wave front aberration generated by optic component were compensated, the divergence angles were compressed, and the beam quality was improved by using the SBS PCM to replace the ordinary reflector in the Nd∶YAG polarex. The pumping energy, lens focal length affecting the beam quality of the SBS polarex were studied.