Using first-principles calculations, the effects of residual gas molecules (H2O, CO, CO2, H-2 and N-2) adsorption on the photoelectric properties of pristine and Zn-doped GaAs nanowire surfaces are investigated. Total energy calculations show that p-type doping surface is beneficial to reduce the damage of residual gases to cathodes and improve the stability of GaAs nanowire photocathodes. After adsorption of gas molecules, the electrons are transferred from surface to adsorbates, leading to a dipole moment pointing from surface to residual gas molecules, which obstructs the escape of electrons and increases the work function of photocathodes. Through Zn doping, the charge transfer between gas molecules and nanowire surface is reduced and the force of dipole moment induced by gas molecules is weakened. Besides, the conduction energy bands shift toward higher energy region and the band gap increased after adsorption of residual gas molecules. Moreover, residual gas adsorption will weaken the absorption characteristic of GaAs nanowire photocathodes.