The infrared radiation features of targets and background are analyzed. A mathematic model of radiant temperature difference transmission of a target and its background is established. The influence of atmosphere on infrared radiation transmission is studied. The influences of water vapor absorption, carbon dioxide absorption and atmospheric scattering on infrared radiation transmission are investigated respectively. The main performance evaluation parameters of a detection system, such as modulation transfer function, minimum resolvable temperature difference and visual range, are calculated. Through simulation, a distorted target image formed by the detection system is obtained. The research result shows that the atmospheric transmission effect can make the target image formed by the detection system to be blurred and cause the contrast between the target and the background to be decreased.