The multiple scattering cluster (MSC) method has been employed to perform a theo-retical analysis on carbon 1s near edge X-ray absorption fine structure of the deuteronacetylene (C2D2) adsorbed on Si(111)7× 7 at room temperature. From the MSC study,it is confirmed that the C2D2 molecule is bonded to a pair of adjacen t Si adatom andSi restatom with C-Si bond length about 0. 18nm. The carbon-deuteron bond is bentaway from the surface andthe CCD bond angle is about 120°. The molecule planetilt slightly away from the surface normal. Compared with C2D2 in gas phase, theC-C bond and C-D bond are elongated by about 0. 03nm and 0. 02nm respectively whenacetylene was adsorbed on the subtrate.