In this paper the carbon K-edge near edge X-ray absorption fine strucure spectra (NE-XAFS) of adsorption system C2H4/Ni(100) are calculated by multiple-scattering cluster method. By a conparison between the theortical results and experimental spectra, the chemisorption geo-metries of this system are determined. The results show that the molecule is adsorbed on the perpendicular bridge site, and the distance between the C atom and the nearest Ni atom is 1.70? while the molecular plane tilting to the surface with 50°.It is found that the interaction between H atom and Ni substrate plays an important role in the formation of the adsorption structure. These results are verified by other ways.