In this paper, the relationship of the threshold power of the fiber source as a function of spectral width of this fiber source and parameters of the polarization maintaining fiber sensing coil was studied based on non-linear stimulated Brillouin scattering theory for the stimulated Brillouin scattering fiber optic gyros (SBS-FOG), which established theoretical foundation for investigating such fiber source conformation and constructing the system of the SBS-FOG.