Abstract: In recent infrared warning experiments, an abnormal phenomena was discovered in which the target radiance increased with the active range. In this paper, how to solve the problem was discussed. Based on the theory of radiance detection equation and by the atmospheric radiance software LOWTRAN, a particular analysis was given about the factors affecting the infrared warning such as the infrared radiant characteristics of the target, the atmospheric irradiancy of the sky background, the irradiancy and the transmittance of the atmospheric path, the detection range, the effective area of the target facing the detector, and the principle. As a result, with the active range increased, the increment of the background radiance was dominant in all influenced factors, so the target radiance was larger and larger. Through the discussion, the abnormal phenomena were interpreted reasonably, and the conclusion was of great significance to develop the algorithm of target extraction for the infrared detective and tracking systems.