We have investigated the electronic structures of Na-incorporated In-rich CuInS2 based on ab initio electronic band structure calculations and X-ray photoelectron spectroscopy (XPS). From the results of theoretical calculations and experiments, for p-type In-rich CuInS2 films, we find that interstitial Na species, except for Na at Cu sites, an very unstable and move easily within the crystal. This indicates that the mobile Na will act as a passivator of donor states, such as In at Cu sites and interstitial In, leading to good p-type conductivity of CuInS2 films. On the other hand, we conclude that the formation of the ionic chemical bonds of Na at Cu sites - S atoms near the surface are energetically favorable due to a decrease in the Madelung energy. This results in the stabilization of ionic charge distributions of CuInS2:Na-Cu with a shift in the energy levels of S 3p orbitals in the vicinity of the Na atoms towards lower energy regions. As a result, the Na incorporation yielded a surface layer of expressed as (Na,Cu) InS2 on p-type In-rich CuInS2 thin films.