Abstract: On account of the present situation that the contrast formula can't be used to calculate target and atmospheric background contrast, the irradiance contrast formula of target and atmospheric background was deduced based on the operating principle of focal plane detector. By calculating the atmospheric background infrared radiation, comparing it with the aircraft infrared radiation data report, and analyzing the contrast, results indicate that: the effect of atmospheric background on detection must be taken into consideration. The atmospheric infrared radiation between 3-5m is very low, target in 3-5m is easier to be detected. The atmospheric infrared radiation near 10m is higher, and the target detection is more difficult. As the detection distance increases, the difference of radiance between the atmospheric path from detector to space and from detector to target decreases, and the contrast increases, which is an unneglectable factor to detection distance analysis.