Abstract: When light is so weak that a single photon is emitted, the single photon can be detected only in photon counting mode. The avalanche photodiode (APD) based on impact ionization effect is used as detector to construct an experimental system of photon counting imaging in low light level environment. The hardware of the system is composed of a single photon counter based on APD, a computer, a low light level illuminometer, two two-dimensional guides, a controller, an obscure box, and so on. The software of the controller is programmed in Altera’s Quartus environment. It controls the movement of the guides. Human machine interface is developed by VC++ software, with the functions of data acquisition and processing, system functions control, and photon counting image display. The photon counti