In this paper the optical properties and spectral reflectance characteristics of multi alkali photocathode were illustrated and spectral reflectance curve of multi alkali photocathode were measured. The shape of spectral reflectance curve is irregular compared with ordinary optical film. The reason is that the cathode layer absorbs light. Interference peak of spectral reflectance curve is the results that the reflection light on the interface between the glass and the cathode layer and the reflection light on the interface between the cathode layer and vacuum interference together. According to interference theory, if two beam light reflected by the cathode film have the optical path difference of even times the λ/2, interference enhancement peak on spectral reflection cure will appear. Similarly, if two beam light reflected by the cathode film have the optical path difference of odd times the λ/2, interference decreased peak on spectral reflection cure will appear. According to the interference theory and peak wavelength on spectral reflection interference curve the cathode film thickness of the super second generation image intensifier can be calculated out. The thickness is about 191nm, and increased by 38% compared with the second generation image intensifier. Determination of thickness simply by observing cathode film color was not an accurate method. Practice has proved that, the method to calculate the cathode film thickness by using spectral reflectance method is simple and effective. If the spectral reflectance were monitored during the process of cathode production, then the thickness of cathode layer would be able to control precisely, and the cathode study would be more in-depth, cathode sensitivity would be further improved.