We have studied a SOA-fiber ring laser and demonstrated its application to a fiber ring laser sensor so far. But in our previous study its accuracy and stability was found to be insufficient. Further investigation by numerical analysis suggested that polarization of the propagating light should be stabilized in the fiber ring laser cavity. In order to control the polarization in the cavity, we have improved the laser. The lasing linewidth has reduced significantly, which shows that the polarization is controlled and stabilized well.