Abstract: In order to acquire a stable narrow-linewidth single-frequency polarized Er-doped fiber laser, the ring fiber laser with a saturable absorber was studied. In the experiments, an un-pumped Er-doped fiber was used as saturable absorber to form an optical narrow-bandwidth filter, a fiber bragg grating with high reflectivity was adopted as coarse wavelength selective component, and a polarization-maintaining optical circulator with high extinction ratio was applied to acquire single polarization laser. The peak wavelength of the Er-doped ring fiber laser can be tuned from 1548nm to 1552nm by pulling the fiber bragg grating. The maximum output power was 55.4mW when the pump power arrived at 275mW. The 20dB linewidth of the Er-doped ring fiber laser was measured to be 3kHz with the fiber delayed heterodyne method. It was found that the polarization of the Er-doped ring fiber laser was very stable for a long term, and the degree of polarization was 99.9%. This narrow-linewidth single-frequency single-polarization Er-doped fiber laser can be used as stable signal source for coherent laser radar.