In this work, the electronic absorption spectrum and nonlinear optical (NLO) properties of the cyclocarbon oxide precursors of C 18 , C 18 -(CO) n (n = 2, 4, and 6), are explored in depth by employing density functional theory (DFT) calculations and wavefunction analysis methods. The influence and regulation of the number of -CO groups on the optical properties of the molecule are focused on, and the essential reason for the significant difference in related properties is unveiled.