We proposed and demonstrated a linewidth compression method of a laser based on stimulated Brillouin scattering (SBS) and a Rayleigh backscattering structure (RBS). The relationship between the output SBS laser linewidth and the input pump linewidth was studied theoretically and experimentally. It is shown that the narrower linewidth of the pump laser leads to the narrower bandwidth of the SBS gain and, finally, the bandwidth of the SBS will tend to its intrinsic value as the linewidth of a pump laser narrower than 10 kHz; then the linewidth of an SBS fiber ring laser would tend to 200 Hz. In order to further reduce its linewidth with low cost, RBS and a simple dual-cavity feedback structure were added and, finally, a ∼75 Hz narrow-linewidth laser with a side-mode suppression ratio of 70 dB was obtained.