The spectral reflectivities of Na2KSb film both with and without the activation layer of Cs-Sb were measured. The measurement results show that the spectral reflectivity of these two films within the wavelength range of 200~700 nm are substantially the same. So it can be inferred that the refractive index of both films are also substantially the same, and both films are substantially no change in its internal constituent. Because the spectral reflectivity is not changed, and the optical path difference of the two films are substantially the same, it can be inferred that the Cs-Sb layer was coated on the surface of Na2KSb, so the activation process of Na2KSb surface is a surface effect. In addition, Na2KSb (Cs) multi-alkali cathode film was analysis by XPS, the results showed that three elements was existed mainly in addition to Na, K, and Sb, but also the presence of CO2 and a small amount of Cs elements. The reason of the presence of C and O on the surface of Na2KSb (Cs) is oxidation in the unsealed process of sample. After 10 second time of argon ion etching, Na2KSb (Cs) multi-alkali cathode film was analyzed by XPS, the results show that the energy spectrum of Cs atoms has disappeared. When continued for 600 seconds time of the argon ion etching, on Na2KSb (Cs) multi-alkali cathode film surface, the spectrum of the Si atoms in XPS spectroscopy was appeared. This showed that argon ion has already etched to the glass window surface. According to the etching time analysis, the thickness of the multi-alkali cathode film surface Na2KSb (Cs) of Cs atomic layers approximately 3nm, and the Cs atomic layer exists only in the surface of the multi-alkali cathode layer, No deep into the multi-alkali cathode film inside. The fluorescence spectrum of multi-alkali cathode with the Cs-Sb activation and without Cs-Sb activation has been measured. The measurement results showed that the peak wavelength of fluorescence spectrum of sample with Cs activation has shifted toward to short directions, while peak intensity of samples with the Cs-Sb has reduced compared with sample without Cs activation. This phenomenon lustrated that not only quantity of transition electron of Na2KSb multi-alkali cathode with Cs activation has increased, but also the energy level of transition electron has increased. This phenomenon can be defined as "volume effect" during the activation process. So during activation process of Na2KSb multi-alkali cathode, the reason of sensitivity increasing is a kind of surface effect and a kind of “volume effect”. Yet this “volume effect” refers to the structure change inside the multi-alkali cathode layer material, rather than surface Cs atoms diffused inside film.