In this paper, density functional theory calculations are used to address the puzzle of CO saturation on Fe(100). Through a comprehensive modeling of possible local configurations, we would be able to figure out a structural model for the CO-saturated Fe(100). The structural model is featured by forming a local structural motif of coupled on-top and hollow CO molecules. In this model, the on-top-bonded CO molecules pertain to critical couplings in the next neighbor spacing, which exclude all couplings below and above the next neighbor spacing. Rather than forming tilted on-top CO on the surface, the critical couplings align the on-top CO molecules in the perpendicular orientation. The new model with the coverage of 0.5ML yields even better X-ray emission and absorption spectra calculated by using density functional theory and shows consistent agreement with all previous experiments.