The carbon 1s X-ray absorption spectra of ethylene adsorbed on the Si (100) surface have been simulated using density functional theory (DFT) in conjunction with the transition-state (TS) method. Finite size cluster models are adopted to describe the spectra of two different interaction modes of C2H4 with the surface, the on-top and bridge adsorption geometries. The accurate modeling of the clusters is tackled by a preliminar optimization of the surface and of the molecules adsorbed on it performed by a periodic slab methodology in the frame of density functional theory (DFT). The suitable clusters for the spectra calculations have been cut out from the optimized periodic structures. Total spectra as well as angle resolved spectra have been simulated. The comparison between the total spectra of the adsorbate models and that of the free ethylene highlights the change of hybridization of the carbon atoms upon the adsorption. The polarized spectra simulated for the ethylene adsorbed in the on-top geometry reproduce correctly the main experimental features and their trend with the change of the polarization while a poor match with the experimental trend is obtained in the case of the bridge adsorption geometry. The results show that a careful analysis of the calculated polarized spectra can provide important information on specific details of the adsorbtion geometries. The methodology employed has proven to be able to describe the K-shell spectra of this kind of systems as well as affordable to be applied to larger adsorbed molecules.