The principle of a polarization interference imaging spectrometer based on Savart prism is presented.The paper analyses the o and e light's clear aperture in Savart prism and the exact expression of the total clear aperture at random incidence angle and position are deduced by ray-tracing method with comparison method.Computer simulations are carried out to give the curve of the effective availability of the clear aperture Varying with incident azimuth angle,specific to the condition that incidence angle is 30 degree,which approximate to experimental Machine.The results will provide a theoretical basis for the study of polarization prisms and the development of polarization splitting apparatus.