Fluorescent spectra between multi-alkali cathode with Cs-Sb surface layer and without Cs-Sb surface layer were compared. It was found that phenomena of peak wavelength of the fluorescent spectrum shifted to shorter wavelength and intensity of fluorescent spectrum became stronger for the multi- alkali cathode with Cs-Sb surface layer. This phenomenon shows that in the condition of a multi-alkali cathode with Cs-Sb surface layer, the surface work function of multi-alkali cathode is reduced, and Na2KSb structure is changed. This means that in the conditions of the same frequencies and the same incident power irradiation, Na2KSb with Cs-Sb surface layer can be excited more transition electrons and obtain higher transition energy levels. So they are able to escape the multi- alkali cathode surface more easily and have higher probability to access the vacuum .This will lead to higher cathode sensitivity and shows that the Cs-Sb surface layer has a surface effect and physical effects. To further increase the sensitivity of multi-alkali cathode, it is needs to improve the performance of Na2KSb layer in addition to further reduce surface work function of the multi-alkali cathode, so that the same power and frequency of incident light can produce more transition electrons and higher energy transition levels, which needs to further process improvements to make Na2KSb material better.