Based on the digital high-sensitivity fluorescence microscope, the ultra-high-sensitivity fluorescence microscope is developed. The system can enhance the fluorescence intensity more than 106 and get the photon sensitive image of ultra-weak fluorescence. The accurate position of ultra-weak fluorescence can be confirmed by image fusion methods. Because the speed of image acquired system is 25 frame/s, the dynamic characters of fluorescence intensity can be investigated. Using the novel microscope to study the apoptosis of MGC-803 cells induced by As2O3, the density of Ca2+ in apoptotic cells obviously increased. Ca2+ is probably one of important roles in the process of MGC-803 apoptosis induced by As2O3.