Polarization mode dispersion (PMD) is a key technology in optical performance monitoring (OPM). A PMD monitoring method based on measuring the frequency of the minimum power of radio-frequency (RF) spectrum is proposed. By cascading a polarization maintaining optical fiber (PMF) with large differential group delay (DGD) and a polarization controller (PC) at the receiver, the PMD can be monitored in low frequency of RF spectrum, and relevant theoretical analysis and numerical simulation are also proceeded. The results demonstrate that this method can monitor first order PMD precisely with low monitoring error in ±3 ps. In addition, it is insensitive to the optical signal-to-noise ratio (OSNR), chromatic dispersion (CD) and transmission rate. Varying the DGD in the PMF at the receiver, this method has different monitoring ranges and sensitivities.