In this work, a systematical investigation on the transformation of tetrachloro-p-benzoquinone (TCBQ) under UV irradiation (at 253.7 nm) in aqueous solution has been conducted through quantum chemical calculations. The UV irradiation at 253.7 nm could induce the excitation of TCBQ to its first excited singlet state, followed by the intersystem crossing to its first triplet state. In aqueous solution, the first triplet state of TCBQ was thermodynamically and kinetically feasible to react with H2O via 1,4-addition, where the addition of OH-to the alpha-beta conjugated system was the dominant step. Interestingly, with the addition of hydroxyl to TCBQ, the dechlorination of TCBQ occurred with the formation of the monohydroxylated product of TCBQ (OH-TriCBQ). The UV photolysis pathway of OH-TriCBQ was similar to that of TCBQ, and the 1,4-addition of OH-to the orthoposition of the hydroxyl was the most efficient pathway. The dechlorination by 1,4-addition of OH-was also observed for OH-TriCBQ. With much larger forward energy barriers, the nucleophilic addition of carbonyl by OH-(i. e., 1,2-addition) might be less important for the UV photolysis of TCBQ and OH-TriCBQ. The findings in the present study may help to understand the transformation of TCBQ in aqueous solution.