A new differential frequency modulation method based on dual-photoelastic-modulator (dual-PEM) and Fourier-Bessel transform is proposed as an key component of dual-photoelastic-modulator-based spectropolarimeter. The dual-PEM are operated as an electro-optic circular retardance modulator. Operating the PEMs at slightly different resonant frequencies ω1and ω2 respectively, generates a differential signal at a much lower heterodyne frequency that modulates the incident light. This method retains the advantages of the existing PEM, and the frequency of modulated photocurrent is decreased by 2~3 orders of magnitude, which can be detected by common array detector. The incident light spectropolarimeter (Q(σ) and U(σ) of Stokes parameters) can be obtained by Fourier-Bessel transform of low frequency component in the modulated signal. The method reduces the complexity of spectropolarimeter measurement system. The basic principle is introduced, the basic equation is derived, and the feasibility is verified through the corresponding numerical simulation and experiment.