Abstract: In order to realize the extension of spectral response of photocathode to short wave, the mechanism of Ga1-xAlxAs/GaAs photocathode, whose spectral response extends to short wave, is analyzed, a new concept of Quasi Energy Gap is presented，the rule that the energy gap of Ga1-lxAs/GaAs widens gradually with the increase of the aluminum composition x is discussed, and the short-wavelength threshold of spectral response for photocathode in different aluminum compositions is calculated. Accordingly, the image tube with wide spectral response was made by increasing the aluminum composition of photocathode material, and the corresponding spectral response curve is given. In comparison with the spectral response of the standard Gen. Ⅲ low-level-light image intensifier, the spectral response curve of the tube we made extends to short wavelength. The experimental result coincides with the theoretical design.