The characteristics of multi-alkali photocathode and its application in low light level image intensifier are introduced, the principle of light induced fluorescence is described, the method to study electron transition probability of Na2KSb cathode film by use of light induced fluorescence is explored, and the fluorescence spectrum of two different cathodes in the states of work and non work are measured respectively. The test results show that, the fluorescence intensity of cathode is proportional to both its electronic transition probability and sensitivity, at the same time fluorescence intensity in work states declines compared to that in non work states. The reason is that part of the transition electrons escape from cathode surface and produce photoelectric emission. These transition electrons do not return to the ground state, therefore they do not emit fluorescence no longer. In this paper the fluorescence spectrum of cathode is also measured excited at different wavelengths of incident light. The results show that, compared with short waves excitation, long waves excitation can obtain higher intensity, which shows long waves excitation can get higher transition electron probability as well; at the same time wavelength shifts between fluorescence peak wavelength and excitation light wavelength is smaller; therefore, the energy loss is small, the photoelectric emission is more favorable. Compared the fluorescence spectrum with multi-alkali photocathode quantum efficiency, it can be concluded that transition electron energy level plays more important role in the process of photoelectric emission between transition electron number and the energy level. As to the multi-alkali cathode, due to electron transition probability of short wave excitation is relatively small, and energy loss of the transition electron diffusion process is bigger, therefore the short-wave quantum efficiency decreases as wavelength decreases. Practice has proved that the photoluminescence is an effective study way of cathode photoemission process, through the study of cathode fluorescence spectrum the cathode photoelectric emission mechanism is further revealed. This provides important reference value for further improving the process and raising the cathode sensitivity.