Abstract: According to the theory of diffraction, the distribution of laser intensity reflected from infrared system is analyzed. The diffraction pattern of spot center intensity in different transmission distance is studied and the relationship between spot intensity and radius is established. The reflection region can be divided into the transmission area, Fresnel diffraction area and Fraunhofer diffraction area. In Fresnel diffraction area, the intensity of spot center change light to dark. The intensity of spot center change violent vibration from 0~1km, and reach peak value between 1~3km, then reduce gradually with the reflection distance increasing. In Fraunhofer diffraction area, the spot center is light spot, around encircle light and dark ring.