We perform a detailed comparative study of Cs/O and Cs/NF3 activation mechanism of negative electron affinity (NEA) GaAs nanowire photocathodes through first-principles calculations based on density functional theory (DFT). The adsorption energy, Mulliken charge distribution, work function, electron affinity, and dipole moment are analyzed. Results show that the stability of Cs/NF3 co-adsorption nanowire surface is stronger than that of the Cs/O model. The incorporation of O or NF3 on Cs-covered nanowire surfaces leads to a sharper decline of surface work function. The NEA states can be attained with both O-2 and NF3. However, during the Cs/O activation process, it needs to strictly control the Cs/O ratio to achieve the maximum photoemission properties of cathode. Moreover, the work function variation of GaAs nanowire photocathodes activated with Cs, O, and NF3 can be explained by the dipole moment model. This work is expected to provide a theoretical guide for improving Cs/O and Cs/NF3 activation procedure for NEA GaAs nanowire photocathodes.