The first step of self-adaptive infrared stealth is the obtention of occlusion background image, this image is a part of the background which is covered by the object when the observer observes this object somewhere. When observing the object in different viewpoints, the background that is covered by the object is different, the object and the occlusion background have a relation of perspective projection. For obtaining the relation between object and the occlusion background, three coordinate systems were established which consist of object′s body-fixed coordinates, viewpoint coordinates and thermal imager coordinates, then the corresponding model between the point on object and the point on the thermal image got by thermal imager was derived by coordinate transformation between these three coordinates. At last an experiment was carried out to verify the model, the result shows that the occlusion background is obtained in different viewpoints by this model. The model lays a foundation of practical engineering application.