Based on the basic imaging theory of the temporally and spatially mixed modulated polarization interference imaging spectrometer (TSMPIIS), a method of interferogram obtaining and processing under polychromatic light is presented. Especially, instead of traditional Fourier transform spectroscopy, according to the unique imaging theory and OPD variation of TSMPIIS, the spectrum is reconstructed respectively by wavelength. In addition, the originally experimental interferogram obtained by TSMPIIS is processed in this new way, the satisfying result of interference data and reconstructed spectrum prove that the method is very precise and feasible, which will great improve the performance of TSMPIIS.