The thickness and composition of multialkali photocathodes are studied with the aid of predication and difinition of reflectance and control of spectral resPOnse and the reflectance on Na2KSb in this paper. If the reflectance and the spectral resPOnse are measured by means of lamp source of 520 urn wavelength, the thickness of photocathodes of the "Super Second Generation" image intensifier must be 120 urn or so, namely after RM3 on reflectance and about 60% S/Smax value. On the basis of (RM1, RM) and (RM, RM2) predicating n and k value and photocurrent before peak (Smax) of spectral response goes steadily up and that after Smax goes linearly down, the photocathode composition is controlled by adjusting evaporation rate of Sb, K and Na evaporator source.