To study the mechanism of long-wavelength infrared (LWIR) polarization detection, a mathematical model is established for the detection system according to its actual detection structure. Various factors in the structure are analyzed theoretically and calculated mathematically by using the related formulas. The influence of the components in the model on the final polarization detection result is obtained and the corresponding methods and measures for improving the detection precision of the whole system are proposed. The mathematical model and theoretical analysis result provide a certain theoretical basis for the study of the whole LWIR polarization detection system and provide referential data for the further experimental verification and practical application.