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In this study, the reaction mechanism and kinetics of dibutyl phthalate (DBP) initiated by hydroxyl ( OH) and sulfate radicals (SO4 −) were investigated at the CAM-B3LYP/6-311 + G(d,p) level through Density Functional Theory (DFT), where the preferred reaction sites of DBP were determined. The calculation results revealed that the reactions of SO4 − with DBP exhibited considerably higher energy barriers than that with OH because of the steric hindrance. For OH, the addition of OH to unsaturated carbons of phenyl ring was kinetically favored with respect to the direct H abstraction from the phenyl ring. In contrast, the reactions of SO4 − triggering formal hydrogen atom transfer from the phenyl ring of DBP were more likely to occur. Interestingly, all SO4 − initiated reactions were not thermodynamically favorable in the gas phase, but the additions of SO4 − to C2, C3, and C4 of the phenyl ring preferred to take place in water. The C2 and C3 were the most reactive sites by SO4 − and OH attacks with energy barriers of 86.5 and 20.1 kJ mol−1, respectively in the gas phase, and the corresponding rate constants were 4.33 × 10−3 and 1.86 × 109 cm3 molecule−1 s−1. In addition, obtained results indicated that both free radicals preferred to attack the butyl chains rather than phenyl ring, which was in good agreement with our previous experimental data. The present work could provide supplementary information on the dual-radicals dependent PAEs degradation.