The dependence of the reflectivity in Stimulated Brillouin Scattering (SBS)on the fluctuations of the pumping intensity was analysized theoretically. The reflectivity stability was close to the pumping stability while the pumping intensity was 1.9 times of SBS threshold. The reflectivity fluctuations were 2.4% less than the pumping fluctuations while the pumping intensity was 5 times greater than SBS threshold.