Since it was first prepared and characterized in condensed phase in 2019, cyclo [18]carbon (C18) has attracted great attention of materials scientists. Currently, reams of theoretical research have been conducted on multiple fronts of C18 and related systems, including aromaticity, vibrational spectra, optical properties, intermolecular interactions, tunneling effect, electron transfer, reaction characteristics, and so on. Our research on the optical properties of C18 demonstrated that C18 exhibits strong ultraviolet absorption and significant third-order optical nonlinearity, which makes it a promising material for ultraviolet filters and nonlinear optical (NLO) applications.