A polarization measurement method employing a Fiber Loop Ring-Down Spectroscopy (FLRDS) system is proposed and experimentally demonstrated. The influence of an Erbium-Doped Fiber Amplifier (EDFA) placed inside and outside the fiber ring on the pulse curve and the number of pulse peaks in the annular cavity is analyzed. A length of 2 m low-gain and low-noise erbium-doped fiber in EDFA is used to reduce waveform distortion and compensate for the attenuation of the cavity. Measurement of polarization angle θ was performed with the FLRDS system, and the change of θ is gained by recording the ring-down time τ of light pulse in the fiber loop. Results indicate that a good linear relationship is met between θ and τ in the range of 0°~ 90°. The sensitivity of the FLRDS system and the correlation coefficient of the fitted curve are 4.05 μs/° and 0.999 6, respectively. The maximum sensitivity error is 0.027 3. Through ten repeated experiments, six sets of data were selected to fit, and the average reproducibility error of the system was 0.030 8, and the repeatability was good, so it can be used in optical rotatory solution measurement, and also provides a reference for fiber radial stress birefringence and polarization state measurement of fiber lasers.