The characteristics of photoluminescence and the measuring principle of photoluminescence of LLL image intensifier were discussed, and the fluorescence spectrum of multi-alkali photocathode of FOP (optical fiber panel) input window was measured. The test results suggest that the fluorescence spectrum of multi-alkali photocathode of FOP window is not a smooth Gauss curve, but a curve with some small interference peaks being added to a Gauss curve, for there are two special light beams in the fluorescence transmitted by FOP window. One light beam is collimated light and the other is the folded light beam at the very moment when the incidence angle is equivalent to critical angle of total reflection. Both light beams have fixed phase difference or optical path. When the difference between the phase differences of the two light beams lives up to the integral multiple of λ, they will interfere to produce constructive interference peak; when the difference between the phase differences of the two light beams lives up to the odd multiple of 1/2λ, they will interfere to produce destructive interference peak. If constructive interference peak is produced exactly at the peak wavelength of fluorescence spectrum, then the measured peak fluorescence intensity is higher than its inherent peak fluorescence intensity; on the contrary, if destructive interference is produced exactly at the peak wavelength of fluorescence spectrum, then the measured fluorescence intensity is lower than its inherent fluorescence intensity. Furthermore, due to the impact of interference, the half peak width of fluorescent curve can not be accurately determined. Hence, the impact of interference factors should be taken into consideration when analyzing the fluorescence spectrum of photocathode of FOP window.