The electronic structures of Si (111) surface have been studied by means of molecular orbital theory of SCF-CNDO with the cluster model treatment. The calculated results show that: (1) The distribution and transfer of net charges in thesurface have localized characters at the atomic orbitals. Also, the T30, T3+ and T3-defect forms do not exist easily in practical system. (2) The electronic structures can be discribed with the stoohastic potential, from which one can find out the adsorption site and learn the adsorption dynamic process of ion and ionic molecules adsorbed on the Si (111) surface. (3) The surface states mainly appear in the region from the top of valence band to the centre of gap, and is strongly localized at the atomic orbitals. The above-mentioned results are consistent with some experimental discoveries.