Tetrabromobisphenol A(TBBPA),a widely used brominated flame retardant,is harmful to human and wildlife in various aspects as an endocrine disruptor.The DFT calculations both in gaseous phase and aqueous phase are carried out to investigate the degradation mechanism of TBBPA.It is found that there exist reductive process with the effect of photoelectron eCB-and oxidative process of ·OH radical attacking three possible sites of TBBPA in the whole reactions,which are different from the degradation mechanism proposed by experiment(Water Res.2012,46(15),4633-4644).According to our calculations,the debromination process is caused by the reduction by photoelectron eCB-rather than ·OH radical attacking C-Br bond.Moreover,the oxidative process is due to ·OH radical attacking three possible sites of TBBPA to form different intermediates.In the debromination process,the reduction by photoelectron eCB-is the most crucial step to reductive degradation of TBBPA,and the formation of tri-brobisphenol A radical(IM3)is the dominant step to form the reductive product tri-brobisphenol A(P1).In the oxidative process,energy barriers both in gaseous and aqueous phase are very close for abstracting hydrogen atom reaction,addition reaction,and substitution reaction of the first step,and it demonstrates that these three different reactions are competitive.By contrast,abstracting hydrogen atom by ·OH radical is the most plausible reaction for forming 2,2-bis(3,5-dibromo-4-hydroxyphenyl)propane radical(IM4),while forming 4-isopropylene-2,6-dibromophenol(P2)and 3,5-dibromophenol radical(IM5)may be difficult to occur because of high energy barriers.However,compared to abstracting hydrogen atom reaction,the addition reaction of the second step is more likely to take place.