A microwave photonics interferometric (MWPI) method for measuring the beat length in high birefringence polarization-maintaining fiber (PMF) is proposed. The approach is based on the radio frequency (RF) interference between the two polarization modes propagating on the polarization axis in the PMF. The period of the RF interference spectral is determined by the optical path difference of the two polarization modes. According to the principle that the phase difference in one free spectral range (FSR) of the interference spectral is 360 degree, the beat length of the PMF can be calculated by recording the RF interference spectral. Two different lengths of PMF have been tested and the measurement results show that the measurement precision is 0.02 mm. The wavelength dependent beat length has also been measured. Furthermore, the measurement precision of this method and the main source of errors are investigated in detail.