The influences of the size of suspended particles in the medium and chemical structural characteristics of the medium on the threshold of the optical breakdown are researched experimentally. The experimental results show that the optical breakdown threshold first rises slowly in accord with the size of suspended particles in the medium, while later it rises sharply (when the diameters of the particles are less than 0.22μm). By purifying the medium, the optical breakdown threshold can be increased; thus the phase-conjugation fidelity, the reflectivity of the energy and the stability of SBS increase correspondingly. Generally speaking, better protection to the inner chemical bond offered by the outer atoms, and less valence electron together with higher ionization energies of the outer atoms will lead to higher optical breakdown threshold of the media.