A simple erbium-doped ring fiber laser based on a high-birefringence Sagnac loop filter is proposed. The Sagnac is composed of a polarization controller （PC） and a polarization-maintaining fiber （PMF）. The effect of PC and PMF on tunable laser is analyzed in theory. In experiment， the single， double and multiple wavelength lasers are obtained by adjusting the PC. The interval of wavelength varies with the length of PMF. The experimental results are in good agreement with the theorefical analysis. In experiment the stable laser output with 20% slope efficiency， 0.016 nm 3 dB line-width， 0.097 nm 30 dB line-width and 40 dB SMSR was obtained.