Infrared polarization imaging technology has the advantages of long detection range and high rate of target recognition. However, the polarization characteristics of targets are easily affected by background radiation in complex environments, which significantly reduces the detection capability of infrared polarization equipment. Based on the polarized Bidirectional Reflectance Distribution Function (pBRDF), this paper establishes a calculation model for the target’s Degree of Linear Polarization (DoLP), comprehensively considering the radiation coupling effect between the target and the background. The variation of the target’s DoLP under two conditions - with and without a strong radiation backplate – is then comparatively studied. Additionally, in order to solve problems of land-based and airborne small-angle detection, simulation research is done to find out how the target’s DoLP is influenced by parameters such as the temperatures and the included angle between the target and the backplate. Research results show that the radiation coupling effect significantly reduces the target’s degree of polarization when the temperatures of the target and the backplate are the same, but it does not change the trend of the target’s degree of polarization, which increases with an increase in temperature. When the temperature of the target and the backplate is 30 °C, 40 °C, and 50 °C, the maximum degree of polarization of the target is 63.7%, 44.9%, and 42.2% of those without a strong radiation backplate, respectively. It can be concluded then that the higher the temperature, the stronger the radiation coupling effect between the target and the backplate, and the greater the reduction of the target’s degree of polarization; and that the strength of the radiation coupling effect is not only related to the temperature, but also to the included angle between the target and the backplate. With the increase of the included angle, the target’s DoLP first increases and then decreases, and the maximum value is obtained when the included angle is about 105°. Therefore, the radiation coupling effect changes the target’s DoLP to a certain extent, thereby affecting the detection ability of the infrared polarization equipment. Finally, through building a long-wave infrared polarization imaging system, the established calculation model of the target’s degree of polarization is verified by experiments, whose results are basically consistent with those of the simulation analysis. Overall, the research results in this paper have certain guiding significance for improving the detection and identification capabilities of land-based and airborne infrared polarization equipment.