In this payer, the sulfur Is and oxygen Is near-edge X-ray absorption fine structure (NEXAFS) spectra of (SO + 2O)/Cu(100) are studied by the multiple-scattering cluster (MSC) method. The cause of the resonances in the NEXAFS spectra of (SO + 2O)/Cu(100) is revealed. The same adsorption structure as in a SEXAFS study is obtained. The analysis shows that the pi* orbital breaks into two levels which shift toward higher energy due to the interaction between SO and the substrate. The low unoccupied orbitals of (SO + 2O)/Cu(100) are examined by a discrete variational (DV)-X-alpha study, which demonstrates the molecular orbital properties of the resonances in NEXAFS spectra of the adsorption system and supports the results of MSC calculation.