An ultra-sensitive imaging system with photocathode sensitivity of 0. 5 cps/mm2 has been developed to obtain the ultra-weak bioluminescence images of a sprounting Mung bean and a living Kunming mouse, and analysis with statistical theory was given for signal testing in these ultra-weak images. In this paper we analyzed the statistical judge to decide whether or not there is signal burying in noise under the condition that both signal and noise obey Poisson probability law, and discussed the five factors which have effects on the judge. With this judge, we have succeed in finding signal in the image of ultra-weak bioluminescence emitted by a living Kunming mouse.