Infrared radiation characteristics are the basis for target recognition in infrared detection systems. Based on the principle of radiation transmission, the infrared characteristics of the target and background in near space were studied. Using the global atmospheric profile to reflect the prior knowledge of the global atmospheric conditions, a set of radiation transmission simulation schemes were designed to study the infrared characteristics of ground targets detected from near space. The MODTRAN model was used for the simulations. The difference between the ground target and background detected in near space was quantified, and we analyzed the optimal transmission band of the sensor as well as the influencing factors of infrared radiation characteristics. The results show that the atmospheric transmittance and infrared radiation difference between the target and background decrease with an increase in the height of sensor and are closely related to the atmospheric conditions. The optimal transmission of the sensor in the range of 3-14.m was obtained; however, the influence of the season, atmospheric visibility, and sensor view zenith angle on the brightness temperature difference between the target and the background cannot be ignored.