Polarization control is of vital importance in optical communications, which can affect the stability and bit error rate of a system. We demonstrate an all-fiber high-speed polarization control scheme based on a bidirectional Sagnac ring. The polarization direction can be accurately controlled by adjusting the phase difference of an electro-optic fiber phase modulator in the ring, meanwhile it is realized that only a single beam port outputs different polarization states. The phase can be adjusted accuratly to 10-3 rad, giving an extinction ratio as high as 30 dB, and the modulation speed can reach 2 GHz. The system has excellent stability, and as the optical circuit consists of simple, low cost components which can be easily integrated, there are good prospects for its application in quantum cryptography and other optical communcations fields.