GaN photocathode has a wide applicaion in ultraviolet detection because of the outstanding performance. GaN photocathode was activated in ultrahigh vacuum (UHV) system by Cs/O, and the reflection-mode quantum efficiency (QE) was analyzed. The QE is 30%～10% corresponding to the wavelength 240～350 nm , and the QE curve is flat. The QE reaches the maximum of 30% at 240 nm. Compared with the abroad result, the QE obtained by us is still inadequate at the short wavelength. The atom arrangement of GaN (0001) was studied. The atom arrangement on the surface was simulated by 3D, and in this way the adsorption of Cs on the GaN(0001) was speculated.