Based on the first principles calculation, this paper systematically studies the adsorption of three gases on the wurtzite GaAs nanowires (100) surface. Compared with CH4 and H2, the surface absorbed with H2O has the lowest adsorption energy, thus it is the easiest to adsorb on the surface. The gas adsorption will result in the increase of band gaps, work functions and surface electron affinity, especially H2O adsorption system with the largest band gap of 1.34eV. The adsorption of these gases produces a dipole moment pointing from the GaAs nanowires to the residual gas molecules. For H2O molecular adsorption, which impedes the electron emission and decrease electrons emission capacity. The surface structures also reconstruct with different degrees. The residual gas adsorption can greatly affect the optical properties of the surface and undermine the performance of GaAs nanowire, particularly H2O adsorption. This work can help to understand the fading mechanism of the performance of optoelectronic devices based on GaAs nanowires.