Abstract: The reflectance and transmission spectrum of the hybrid Sagnac fiber optic interferometer which is consist of single mode fiber coupler and polarization maintain optical fiber were investigated and the modulation character of the spectrum was illustrated. A simple measurement method for the beat-length of polarization maintain optical fiber was proposed based on the character of the transmission spectrum. And the optimization parameter for the measurement based on the error analysis was obtained. The experimental system was set up and the experimental results agree well with the theoretical prediction. The result shows that this technique has the advantage of simple in principle and high precision, the accuracy of the beat-length measurement is better than 0.01mm.