We report a density functional theory (DFT) study regarding the effects of Na on field emission of single-wall carbon nanotubes (SWCNTs). Specifically, a comprehensive examination is carried out to investigate the effects of Na adsorbed on capped C(5,5) armchair SWCNT tips. A capped (5,5) carbon nanotubes model with different Na atoms adsorbed is built, and the adsorption energy, ionization potential, local density of states, Mulliken charge population of the model with and without the applied electric field are calculated and analyzed. The Mulliken charge population analysis shows that the charges are redistributed and accumulated on the tip. Under an applied electric field, the number of Mulliken charges that transfer from the carbon nanotube body to its tip increases with the increase of the number of Na atoms. The local density of states at the Fermi level increases with the adsorption of Na atoms. These results reveal that the field emission properties of carbon nanotubes can be enhanced by the adsorption of Na atoms.