For meeting rigor demands of modern light communication technology on each technic target of polarizer, a mathematically analytical expression of polarizing prism is deduced by means of theory of membranous layer when incident light is vertical to the crystal plane. Each physical parameter of affecting polarizing prism transmission technic target is analysed quantitatively. Computer simulations show that influential range of crystal thickness on transmission of polarizing prism associates with transmission of air inter layer. When parameters, such as crystal refractive index etc. are endowed with given values, for thickness of air inter layer h=26.2 μm and 27.4 μm, influential ranges of crystal thickness on transmission of Glan-Taylor prism are 5.934% and 7.034% respectively, and influential ranges of crystal thickness on transmission of Glan-Foucaul prism are 1.24% and 8.893%. Transmission of polarizing prism is affected by crystal thickness, air inter layer thickness and configuration angle. This theoretical research overcomes the shortcoming of previously transmission formulas of polarizing prism. They cannot analyse influence of crystal thickness on light transmission of polarizing prism quantitatively. The theory has importantly applied worthiness to polarimeter' design.