A scheme of microscope polarization interference imaging spectrometer (MPIIS) based on a Savart polariscope and its basic principles are introduced, and the relative expression of depth of field of MPIIS is derived with the geometrical-optics method. The relationship between the depth of field and the spectral resolution is given. It proves that the existence of the depth of field renders the spectral resolution value unfixed, but changing continuously within a certain range. The influence of depth of field on resolution value of a MPIIS is analyzed through computer simulation. All of this provide important theoretical bases and practical guidances for realizing real-time measurement, decreasing influence of depth of field on spectral resolution, and the study, design and engineering of novel polarization interference imaging spectrometers.