In order to achieve the target detection and imaging in ultra weak light environment,the solid state avalanche photodiode in Geiger mode is used as the detection unit to build the photon counting imaging experimental setup at the illumination of 10-3 lx.The photon counting frequencies of the gray level images can be obtained.The photon counting imaging gray level model that shows the relationship between photon counting frequency and gray level is developed based on the experimental results and the statisti...