Considering that illumination using the light source is required to monitor the activation progress of negative-electron-affinity GaAs-based photocathodes, it is important to understand if and how the illumination affects the activation. To improve the photoemission performance of GaAs photocathode, epitaxial GaAs samples are Cs/O2 activated and recesiated under illumination of monochromatic light of blue (460 nm), green (532 nm), and red (633 nm) with approximately the same incident photons, and halogen tungsten lamp as white light source, respectively, to induce photoemission. The performance characteristics including quantum efficiency and photocurrent degradation among the samples treated by different illumination conditions are compared to investigate their photoemission capability and stability. The results show that GaAs photocathodes activated and recesiated under illumination of monochromatic red light, can obtain higher quantum efficiency, and better stability. This work provides a favorable illumination condition during activation for obtaining satisfactory capability of GaAs-based photocathodes, which will be propitious for applications as spin-polarized electron sources.