The principle of beam splitting of interference imaging spectrometer based on Savart plates is presented. The propagation track of light wave in crystal and the exit aperture is analysed by combining wave normal-tracing method with ray-tracing method at random incidence angle and azimuth angle. The relationship among exit aperture, incident angle, incident position and azimuth angle is deduced in detail. The requirements that the propagation track of light remains in Savart plates and would not exit from the top surface, bottom surface and side are discussed in detail. The area and the position of exit aperture are simulated by computer, which proves the correctness of the deduction for normal incidence of light. It is shown that the lateral shear of single Savart plate restricts the boundary of clear aperture, and the area of the clear aperture is only 85.73% against the total incident surface. The parameter of experimental prototype is introduced and the clear aperture is in detail analysed and discussed by the method mentioned above. The results show that the accurate and the approximate values of exit aperture area of experimental prototype are greatly different, and the position of exit aperture are shifted into the lower right, which may reduce the image quality and even cannot generate the double-beam interference fringes in some specific areas. The effective clear aperture as a function of azimuth angle is also presented. It shows that the used clear aperture area is between 0.8005 and 0.8547 while changes from 0 to 2, in order to match the conditions that the value of area availability decreases to 0.6976 when the light always propagates inside the Savart plates. The article shows that the change of clear aperture caused by crystal birefringence phenomenon cannot be ignored when selecting the instrument aperture stop and parameter of interference imaging spectrometer. The clear aperture of the two beams, o-light and e-light, which propagate in Savart plates should be calculated respectively and then they are used to determine the last clear apertures of plates. This study provides a theoretical and practical guidance for study, design, modulation, experiment and engineering of interference imaging spectrometers.