Adsorption of organic molecules on semiconductor surfaces has been attracting a growing attention for its importance in emerging technologies. Since the properties of the resulting materials are largely dependent on the organic/semiconductor interface, fundamental research on the covalent bonding of molecules with the surface can provide useful information. Problems that have been addressed include the structure of the resulting systems and the spectroscopic measurements, often in concert with theoretical calculations, can assess the orientation and geometry of the molecular adsorbate. NEXAFS spectroscopy is widely used to characterize adsorbate structures on surfaces since it allows to investigate the adsorption mode as well as the extent of the adsorbate-substrate interaction, through the comparison between the spectra of the free and adsorbed molecules. Here we present a DFT simulation of the NEXAFS spectra of ethylene adsorbed on a regular Si (100) surface, considering several adsorption models. Cluster models have been employed to mimic the molecule-surface system for the spectra simulation. Surface models with and without the adsorbed molecule have been optimized by means of periodic slab DFT calculations. Angle dependent spectra have been also calculated and compared with available polarization dependent NEXAFS experiments, revealing the suitability of this technique to derive the adsorption configuration of the molecule on the surface.