The principle of beam splitting of modified Savart polariscope in the static large field of view polarization interference imaging spectrometer(SLPIIS) is described. The reflection and refraction in every uniaxial of modified Savart polariscope are analyzed， with the incident plane coincident or perpendicular to the principle section of the left plate，based on the electromagnetic boundary conditions. Formulas for the transmittance of the modified Savart polariscope are given as functions of wavelength of the incident light，angle and thickness of the half-wave plate，which are analyzed through computer simulation with different half-wave plates. It is proved that the modified Savart polariscope has the advantages of having both large field of view and high flux，and can be used as high performance lateral shearing beam splitter in SLPIIS. This theoretical study will provide guidance for the experimental study of new polarization interference imaging spectrometer.