Multi-alkali photocathode in super generation image intensifier is different from previous multi-alkali photocathode between the production processes, so the photoelectric emission characteristics are different from previous multi-alkali photocathode. In this paper, through the measurement of multi-alkali photocathode spectral reflectivity and transmissivity, according to the law of conservation of energy, cathode spectral absorption rate was obtained. Spectral absorption rate indicates that, only when the photon energy greater than 1.333 eV, cathode absorption rates began to increase quickly. The cathode spectral absorption shows that cathode will not absorb any photons if light incident photon energy is less than the absorption limit, i.e. 933 nm wave absorption limit. In the cathode surface electron affinity further reduced circumstances, cathode photoemission long wave theory threshold is determined by long-wave absorption limit. In the electronic transition process after absorption of a photon, transition energies increase less than the absorption of the incident photon energy, i.e. the presence of an “energy loss”. The higher energy of a photon is, the higher electronic transition energy level is, the more energy loss is. At the same time, the higher energy of a photon is; the higher-energy level transition electron, the lower electronic transition probability is. Photocathode quantum efficiency is determined by the absorption rate, the transition probability and transition level, energy loss of diffusion process and other factors, thus photocathode quantum efficiency is in the presence of long wave threshold and shortwave threshold also. Photocathode quantum efficiency in 587 nm reaches the maximum value, after that decreases with the photon energy increases, when 3.6 eV, the quantum efficiency is reduced to zero. Cathode in photon energy of 3.6 eV the absorption coefficient is still high, but due to the electronic transition probability is low, while the electron diffusion process of energy loss is big, thus in spite of cathode on lower wavelength has high absorption coefficient, but the quantum efficiency is still low. Therefore on shortwave, cathode absorbed shortwave photon energy is converted into a photoconductive, lattice vibration and other forms of energy; the photoelectric emission utilization rate is very low.