Aiming at the issue of the ground targets infrared stealth, the radiometric resolution and the spatial resolution of the sensor system were deduced, including the effects of the earth surface and the atmosphere effect. Radiometric resolution of the sensor for different background was analyzed taking these factors into account: atmospheric scattering and radiation transfer, aperture and transmittance of optical system, infrared detector directivity, signal to noise ratio and the infrared radio intensity of the ground target and the background. The spatial resolution of the Infrared imaging system was also calculated for diffraction-limited infrared system. Based on the two aspects, a method of calculating the detection capability of the space infrared imaging system to the ground target was developed. In the calculation, factors are taking into account such as average temperature difference of the target and background, the temperature of the background, minimum resolved temperature difference, apparatus transmittance and the atmospheric transmittance and the size and geometric of the target. So the target can be or not detected not only depended on the characteristics of the target and the background but also related to the infrared imaging system. Otherwise, for the Identifying of ground targets for space infrared system, it depends not only on the similar degree between targets and background but also for the particular characteristics of the target.