In the paper, the phase conjugation effect and the threshold effect of SBS inducing the reducing of the divergence angles of back-SBS were discussed in the theory. In the experiment, the wave-front aberration was compensated, the beam quality was improved, and the divergence angles were compressed by using the SBS PCM in the Nd:YAG laser. The pumping energy, lens focal length and SBS medium affecting the beam quality of the amplifying back-SBS were studied, the beam quality was improved in evidence by using SBS system to replace the general holophote.