Fluorescence spectrum of Na2KSb(Cs) multi-alkali photocathode of the GenⅡ+ image intensifier was measured with laser of 785 nm wavelength as the excitation source. It was found that the fluorescence spectrum is not a smooth Gaussian curve, but a Gaussian fluorescence curve superposition of small jagged peaks with a certain frequency interval. Experimental and theoretical analysis show that the small fluorescent jagged peaks on the curve are actually a fringe with relationship of the structure of the GenⅡ+ image intensifier. Pitch between two adjacent interference fringes is proportion to the product of peak wavelength of two adjacent interference fringes, and inversely proportion to the proximity focusing distance of the GenⅡ+ image intensifier. Modulation of interference fringes of fluorescence spectrum is inversely proportion to the thickness of multi-alkali photocathode. By measuring the pitch of two adjacent interference fringes and modulation of Na2KSb(Cs) multi-alkali photocathode of the GenⅡ+image intensifier, the proximity focusing distance and the thickness of Na2KSb(Cs) multi-alkali photocathode of the GenⅡ+ image intensifier can be measured. The results provide an effective means to further increase photocathode sensitivity and resolution of the GenⅡ+ image intensifier.