The Cs-only and Cs-O co-adsorption process of Zn-doped GaAs nanowire surface with negative electron affinity are investigated utilizing the first-principles calculations based on density function theory. The 1Cs, 2Cs, 3Cs and 4Cs adsorption models are built to simulate the process of Cs-only adsorption, while 3Cs/1O, 3Cs/2O and 3Cs/3O models are established to study Cs-O co-adsorption process. The adsorption energy, work function, Mulliken charge distribution and dipole moment for each adsorption model are discussed, respectively. During the Cs-only adsorption process, as increasing Cs coverage, the adsorption energy increases accordingly, indicating that the stability of Cs-only adsorption model is gradually weakened. After O adsorption on Cs-covered GaAs nanowire surface, the reduction of adsorption energy means that O incorporation can make the surface adsorption structure more stable. For Cs-only adsorption models, the work function obviously decreases owing to first dipole moment induced by Cs. However, excessive Cs adsorption on the Zn-doped nanowire surface will result in the 'Cs-kill' phenomenon. After O atom incorporation, the work function of Cs-covered nanowire surface further decrease, owing to the dual-dipole caused by Cs, O adatoms and nanowire surface atoms. The lowest work function can be obtained for 2O co-adsorption model, indicating the optimized atomic ratio of Cs/O is 3:2. These calculations provide an attempt to explore the adsorption mechanism of Cs-O on nanowire surfaces, and the results are expected to be verified by the experimental results in the future.