An ultra sensitive imaging system was developed and used to obtain the ultra weak builuminescence image of a living Kunming mouse, and statistical analysis was used for studying photon frequency distribution. In this paper, for the first time we use X 2 ruler to calculate the signal photon and background photon distribution parameters for the image processing. It has been proved that this method is effective in processing the ultra weak bioluminescence images. The S/N ratio is increased and the image quality is improved.