The atmospheric background is analyzed and the atmospheric radiation intensity and transmittance at different pitch angles are calculated. By analyzing the radiation principle of a high dynamic target in atmosphere, both the calculation model of the infrared characteristics of the target affected by high temperature gas and the calculation result of the infrared radiation intensity of the target in different wavebands are given. The Signal to Clutter Ratio (SCR) and detection range are calculated by formulas and are analyzed. Finally, the conclusion that the detecting ability is limited is reached.