The Savart polariscope is the key spectroscopic component of the static polarization interference imaging spectrometer (SPIIS) that we have developed. The principle of beam splitting of Savart polariscope is described. This paper mainly studies the reflection and refraction of light incident from one medium to another when incident plane and the principle section of the left plate are coincident or perpendicular, according to electromagnetic boundary conditions. The transmission coefficient of each interface is deduced, and the total transmittance is given as a function of angle and wavelength of the incident light which is analyzed through computer simulation. It is proved in theory and by experiment that the Savart polariscope has the advantages of both high flux and large field of view, and can be used as high performance lateral shearing beam splitter in static interferometer and SPIIS. This will provide the theoretical basis for the development of new polarization interference imaging spectral systems.