Using the projected augmented wave potential based upon the density functional theory within the gradual gradient approximation approach, after the optimization of wurtzite structure GaN, the affinity variation of Cs atoms adsorbed on GaN(0001)A surface was calculated, which proving that an effective GaN-Cs dipole layer was formed, and be good for electrons escaping form the substrate. The electronic structure of adsorbed Cs and O on GaN(0001)A surface was also calculated, which pointed out the bonding of Cs and GaN substrate. Furthermore, the internal quantum efficiency of reflect photocathode of GaN material on various minority carrier diffusion length were derived from dielectric functions theoretically. The calculated results demonstrate that GaN(0001)A surface is an excellent emitter for visible-blind photocathode, and the efficiency at 254nm can reach up to 60%, far more than other alkali halide UV photocathodes.