In combination with Plank's radiation law, a dual-band temperature measurement method is used to analyze the temperature and infrared radiation of a distant high-altitude target. A method for calculating the projected area of the target is proposed. Because the target is detected at a long distance, a model for calculating the atmospheric attenuation in arbitrary direction of two sides taking the curvature of the earth into account is presented. It is used to analyze the attenuation of infrared radiation in both 3 to 5 μm and 8 to 12 μm bands at different distances. The experimental result shows that in the high altitude about 10 km away, the measuring distance of the infrared radiation from the target can be increased from a few tens of kilometers in low altitude to hundreds of kilometers. This provides a theoretical and technical support for the further analysis of the radiation characteristics of targets.