Stimulated brillouin scattering (SBS) is one of important nonlinear effects in fiber communication system or optical fiber sensing system. SBS can be brought easily in fiber for its low threshold, inducing power loss of incident laser in fiber system which is engaged as signal carrier, damaging laser source for backward scattered light caused by SBS, limiting laser power and transmitting distance of fiber system. Basing on the theory of SBS, the relation of SBS threshold with modulating frequency of laser source, linewidth of laser source, length of fiber and coefficient of loss is analyzed. The SBS threshold of 19.5 km G.653 fiber is measured by experiments, and the results accord with the theory calculation.