The 3rd generation low-light-level image intensifiers should be aged for 100 hours before its normal use. In order to know the influence of ageing processing on GaAs photocathodes, five 3rd generation low-light-level image intensifiers were aged with the life testing instrument of low-light-level image intensifier in an experiment. With the spectral response testing instrument, the intensifiers were measured for totally 8 times to get their spectral response respectively before they were aged and in a half year after aged, and to calculate the integral sensitivity according to the spectral response curves. Based on the fluctuating spectral response curves and the varying integral sensitivity, it was indicated that the aged intensifiers up to standard had more stable photocathode sensitivity, smaller decrease in their spectral response curves, while those not up to standard had more obvious decline as a whole in their spectral response curves. Additionally, the threshold wavelength of all intensifiers was moving toward shortwave. The degeneration of GaAs photocathode resulted from the instability of the Cs-O layer on GaAs photocathode surface. During the ageing processing, the lack of a longtime light radiation on Cs-O layer, the widening surface barrier and the decreasing escape probability led to less photoelectronic emission and lower sensitivity. Moreover, the destruction of dipole layer resulted in smaller bending of surface band and higher vacuum level, so that the electrons in impurity level could not escape and the threshold wavelength moved toward shortwave. Thus the ageing processing played a role of picking out the 3rd generation low-light-level image intensifiers to get rid of the products not up to standard and to put the photocathodes of products up to standard into a relatively stable random failure period.