Operating range is the main performance index of infrared detection system.The effect of atmospheric transmission medium on mid-infrared waveband was studied.The infrared radiation of typical flying target in atmospheric transmission was calculated.The varieties of operating range with relative humidity and visibility were analyzed.The results show that operating range decreases with the increase of relative humidity and increases with the increase of visibility.For the target temperature of 800 K,operating range in 8~12 μm is greater than that of 3~5 μm when visibility is below 8 km.Operating range in 8~12 μm is less than that of 3~5 μm when visibility is good.The method can provide a reference for design and performance evaluation of infrared detection tracking system.