The beam splitting and polarizing principle of modified Wollaston prism are presented. This paper analyses the propagation track of light wave in modified Wollaston prism by wave-normal-tracing method at random incidence angle and the formula of the path difference between e and o light waves are deduced. Computer simulations are carried out to give the curve of the optical path difference varying with incidence angle, incident azimuth angle, structural angle and wave length. The results will provide a theoretical basis for the study of polarization prisms and the development of polarization splitting apparatus.