Aiming at the elements of infrared polarization imaging system based on metal wire-grid polarizer, a lot of affecting factors such as target, atmosphere, sky, polarizer reflect and radiate itself should be take into accounted, which is favorable for analyzing the infrared polarization radiation constitutes entering into the imaging system. Based on the bidirectional reflectance distribution function model, the paper establishes transmission equation of infrared polarized radiation that includes reflection and emission, and we deduces the mathematical model with respect to a lot of factors of the degree and the angle of polarization. By simplifying the formula of degree of polarization reasonable, the simulated curves of degree of polarization are consistent with the measured date in references. The result of this paper can afford gist theory and techno-gsustain for improving the detection capability of infrared polarization imaging system.