A new differential frequency polarization modulation method based on triple-photoelastic-modulators (triple-PEMs) is proposed to overcome the shortcomings like that phase locked frequency is difficult to be measured by common array detectors and the polarization of polychromatic light is hard to be obtained. With the method, the tandem PEMs are operated as electro-optic circular retardance modulators at slightly different resonant frequencies ω1, ω2 and ω3 respectively, and they generate differential signals at much lower heterodyne frequencies (0, ω1-ω2, 2ω1-2ω3) by modulating the polarized components of the incident light. Then, the Stokes parameters S0, S1 and S2 can be obtained simultaneously by the lock-in amplifier circuit with one measurement. The basic principle is introduced, basic equations are derived, and the feasibility of the method is verified through the corresponding numerical simulation. The preliminary analysis indicates that the method not only retains the advantages of the existing PEM-based polarimetry, but also decreases the frequency of modulated photocurrent by 2-3 orders of magnitude(10-500 Hz). Moreover, its signals can be detected by a common array detector. This method has potential applications to the high precision polarization imaging technology.