Quantum chemical calculations studies were performed to investigate the underlying photodehalogenation mechanisms of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and polybrominated dibenzo-p-dioxins and dibenzofurans (PBDD/Fs), as well as Mg2+ effects. The bond dissociation energies of C-Cl/Br bonds, electronic absorption spectra, vibrational frequencies, and NBO charges of PCDD/Fs and PBDD/Fs were calculated by density functional theory and time-dependent density functional theory. Results show that the photodehalogenation reactivity of PCDD/Fs and PBDD/Fs increases with an increase of halogenation degree. Halogen atoms attached to the benzene ring with more halogens substituted have higher photoreactivity. For PCDD/BDDs, the photoreactivity of halogens at 2 and 3 positions are higher than that of halogens at 1 and 4 positions. For PCDF/BDFs without halogen substitution at 9 position, the order of photoreactivity of halogens follows: 2, 3 > 1, 4 position. However, for PCDFs/BDFs with halogen substitutions at 1 and 9 positions, the order of photoreactivity of Cl/Br atoms follows: 1 > 2 > 3 > 4 position. 1,2,3,4,6,7,9-H7CDD/BDD has been predicted to be the main photodehalogenation products of O8CDD/BDD. However, by the presence of Mg2+, the bond dissociation energies of C Cl/Br bonds of O8CDD/BDD at 4 and 6 positions are drastically reduced and the main photodehalogenation products are changed to 1,2,3,4,6,7,8-H7CDD/BDD. Furthermore, the presence of Mg2+ significantly reduces the bond dissociation energies of C-Cl/Br bonds of O8CDF/BDF at 4 and 6 positions, but has no effects on the main photodehalogenation products. These findings provide an insight into the evaluation and prediction of photochemical behavior of persistent organic pollutants.