Abstract: The air gap of polarization prism is influential to the transmissivity of this type of polarizer.Optimized design of the thickness of air gap is very useful to improve e-ray transmission in polarization prism.By means of the Glan-Taylor prism,based on the theory of Fresnel formula and multiple-beam interference,the calculation formulum of e-ray transmissivity in air-spaced prism is put forward.The law of the e-ray transmissivity varing with the air gap thickness is analyzed so that the optimum air gap thickness can be obtained to design an optimum polarizing prism.