There are few reports on the relationship between the biological activity and structure of secondary metabolites from Dictyota dichotoma. In this work, the geometric and electronic structures, infrared spectra, ultraviolet spectra, 13C and 1H NMR spectra of 16 xenicane-type diterpenoids extracted from Dictyota dichotoma were studied by the density functional theory ωB97XD/6-311+G(2d,p) method. The analysis of the reactivity indices was carried out via the conceptual density functional theory. Furthermore, a pharmacodynamic evaluation was performed using ADME/Tox. The geometric structure analysis found that all 16 diterpenoids had the same unsaturated, branched chains and could be divided into two categories according to the ring size, including nine-membered and eight-membered rings. A hydroxyl group on the lactone ring reduced the stability of the compound. In contrast, a hydroxyl group on the parent ring had little effect on the compound's stability. The electrostatic potential results preliminarily predicted active sites for nucleophilic/electrophilic reactions. The fitting results of infrared and nuclear magnetic resonance(NMR) data showed that the theoretical values obtained by this method were consistent with the experimental values. The UV-visible absorption spectra showed that the solvent effects caused different redshifts of the absorption peaks. The absorption strength was enhanced. The sixteen diterpenoids displayed a strong absorption peak in the range of 180―200 nm, but the compound containing a carbonyl group presents a weaker absorption peak in the field of 200―240 nm. Compounds 9―11, 14, and 15 have better stability and reactivity with lower chemical potentials and higher electronegativity, electrophilic index, and hardness values. The local reactivity descriptors further identified nucleophilic/electrophilic reaction sites for the sixteen compounds. Finally, the pharmacodynamic evaluation results predicted that compounds 2, 4, 8, 11, and 14―16 have optimal drugability. The theoretical re
