Abstract: Infrared detection of the ground target was greatly determined by the radiation contrast between the target and its background. The influence of emissivity of target and temperature difference on radiation contrast was briefly discussed. Factors on buildings and cement floor temperature were also analyzed. The model that computed temperature was built on the basis of finite difference method and realized by MATLAB. The target-background radiation contrast at 8-14m was calculated on the basis of the simulated temperature. It turns out that the radiation contrast of the same target and background changes significantly under different conditions. Variation trends of radiation contrast of the same target and background when observed in different azimuths are also presented.