Due to the fact that the photoelastic modulator (PEM) interference signal frequency ranges from hundreds of megahertz to several gigahertz, and the array detector can not be applied to such high responded frequency, the application of PEM in spectral imaging is greatly limited . To solve this problems, a method is proposed in this paper to produce two PEMs of slightly different resonant frequencies with f1 and f2 which are applied to modulate the inputting optical signal. This method operates two PEMs of slightly different resonant frequencies f1 and f2 respectively, and the incident light can be modulated by the frequency difference of two PEM system. Thus, the interference signal contains low frequency modulation components contains the information of the incident light, and low frequency modulation component consists of a series of frequency-multiplier signals whose fundamental frequency is (f1-f2)/2. This low frequency modulation components can be detected with normal detector. After filtering the direct current component and high frequency signal, the incident light spectra can be obtained by the corresponding calculation. Since the difference frequency can be 2~3 orders of magnitude lower than the original resonant frequency of PEM, it can offer more time to array detector to process the inputting signal. What’s more, it does not require the same path difference and resonant frequency for the two PEMs, which can reduce the difficulty to PEM modulator design and control.