Abstract: Generalized gradient approximation projector augmented wave method based on first-principle in the frame of density functional theory (DFT) were put forward. The slab model of GaAs (110) crystal plane was adapted for calculating adsorption system on basis of the optimization of bulk GaAs structure. Three types of adsorption system including specific adsorption sites, total binding energy, and adsorption electronic structure were taken fully into account with adsorbate quantity of =0.5 monolayer (ML) sole Cs,=0.5 ML sole O, and =1.0 ML Cs, O, respectively. The comparison results of calculated total binding energy and projected maps of electron density show that when adsorbates of Cs, O reach to=1.0 ML, they don't form local domain of competitive chemical adsorption, while they form a compound uniformity phase of cooperative chemical adsorption. Considering electronic dipole correction in the calculation, the work function of the three adsorption systems were 4.423 eV, 5.749 eV, 4.377 eV, the method and mechanism for improving and maintaining photoemission characteristics in GaAs photocathode preparing technology were further obtained.