Possible configurations and optical properties of a cucurbit[7]uril (CB7) and riboflavin (VB2) inclusion compound were investigated by density functional theory (DFT).Acomparison was made among the different structures by an analysis of the interaction between the host and guest molecules. The results indicate that riboflavin sits on top of the cucurbit[7]uril molecule with a couple of hydroxyls inserting into the cavity to formH-bonds.We verify theoretically that the inclusion reaction is an exothermal reaction, which supports the findings from the experiment very well. The sequence of stability for the possible inclusion complexes is strong evidence that the interaction between the guest and host molecules and the distortion of the guest and host molecules both contribute to the stability. The bonding energy and stability should be distinguished conceptually. The time-dependent DFT method was used in the excited state calculations to explore the photochemical properties of the inclusion compounds. The component difference for the excited orbitals in the different inclusion complex configurations was compared theoretically. The results show that cucurbit[7]uril causes a red-shift absorption and fluorescence quenching for riboflavin after host-guest inclusion, which strongly support the experimental data.