Abstract: The contrast between a ground target and its background under infrared illumination is very important for target detection, identification and tracking, as well as IR stealth and simulation of infrared signature. The theory model for a ground target irradiated by an artificial infrared illuminator is established. According to the definition of contrast, the basic calculation for the contrast between a ground target and its background under infrared illumination is derived; the factors affecting the contrast between the target and its background are calculated and analyzed. The results show that the greater effect on the contrast is observed when the target and its background are illuminate at a shorter distance with greater power and longer time. For illuminators at different distances, with the change of incident angle, the effect of the illuminator on the contrast becomes greater.