Research on polarization characteristics of objects has become indispensable in the field of target detection. Though widespread studies on applying polarization to target detection and material identification exist, theoretical descriptions have varied widely in accuracy and completeness. Incomplete descriptions of polarization characteristics invariably result in poor demonstration of changes caused by macroscopic influence factors. For objects that are of finite surface, a comprehensive model is built to analyze the polarization characteristics of their thermal emission. With the Stokes theory and the superposition principle of light waves, the relation between the degree of linear polarization and the spatial geometrical parameters, such as the detection distance and the shape of objects, is discussed in the long-wave infrared range in detail. This model can be applied to analyze the linear polarization characteristics among different materials.