Based on the detection results of quadrupole mass spectrometer （QMS） on ultra high vacuum system， the adsorption systems of residual gases adsorbed on Ga_0.75Al_0.25N（0001） surface were built. Based on density function theory（DFT）， the adsorption of residual gases on Ga_0.75Al_0.25N（0001） surface was calculated. Results show that residual gases enhance the surface work function. The changes of the work function were caused by the dipole moments between the adsorbates and the surface. The dipole moments caused by residual gases pointed to the surface from bulk， which was harmful for electron escaping from surface into vacuum. At the ultraviolet waveband， the absorption coefficients of surfaces with foreign gases were smaller than that of the Cs adsorbed surface.