DFT calculations in both gaseous phase and solution are carried out to investigate the degradation mechanism of tetrabromobisphenol A (TBBPA) under photocatalytic UV/Fenton conditions. It is found that there exist reductive process and oxidative process. Our calculations show that the reductive process is caused by a conduction band (CB) photoelectron e CB − to make the C–Br bond broken, while the oxidative process is due to ·OH radical attacking three possible sites of TBBPA to form different intermediates. In the reductive process, the reduction of TBBPA by a photoelectron e CB − is coupled with C–Br bond cleavage, and the formation of tribromobisphenol A radical (IM3) is the rate-determining step to form the reduction product tribromobisphenol A (P1), where in the experiment (Zhong et al. Water Res 46(15):4633–4644, 2012), mechanism being proposed as ·OH radical attacking C–Br bond. In the oxidative process, abstracting hydrogen atom by ·OH radical is the most plausible reaction to form 2,2-bis(3,5-dibromo-4-hydroxyphenyl)propane radical (IM4). IM4 can receive a conduction band electron e CB − to yield 2,2-bis(3,5-dibromo-4-hydroxyphenyl)propane carbanion (IM4′), followed by a C–C bond breaking reaction, resulting in the formation of P2 and 3,5-dibromophenol carbanion (IM5′).