In this paper, by comparing the fluorescent spectrum of the multi-alkali cathode with a surface layer of Cs-Sb and that of the multi-alkali cathode without a surface layer of Cs-Sb, we find that the peak wavelength of the fluorescent spectrum of the multi-alkali cathode with a surface layer of Cs-Sb moves towards the SW, and the fluorescent peak is enhanced. This phenomenon shows that after a Cs-Sb surface layer is produced on the multi-alkali cathode Na2KSb basic layer, the work function of multi-alkali cathode decreases; besides, the structure of the Na2KSb basic layer is changed. This means that with the incident light of the same power and the same frequency, the Na2KSb basic layer that has gone through the Cs-Sb surface processing can produce more transition electrons, with higher transition levels, more possibilities of escape from the surface, and greater sensitivity of cathode obtained. Therefore, after the Na2KSbcathode film has experienced the Cs-Sb surface activation, the spectral response increases not only because the work function on the surface decreases, but also because the internal energy band structure of the Na2KSb film is changed. After the Na2KSb cathode film has gone through the Cs-Sb activation, there will be a layer of Cs-Sb film on the surface. Stress existing between the two films result in lattice distortion of the Na2KSb film. As a result, the energy band structure is changed. To further improve the sensitivity of the multi-alkali cathode, we need further reduce the work function of the multi-alkali cathode, and further perfect the performance of the Na2KSb basic layer, so that the incident light with the same power and frequency can generate more transition electrons with higher transition levels, which requires constant process improvement of the process and the Na2KSb material performance.