Polybrominated diphenyl ethers (PBDEs) have received special environmental concern due to their potential toxicity to humans and wildlife worldwide, however, it is difficult to reveal their dominant photochemical degradation pathways by experiment. We explored the reaction mechanisms of photochemical degradation–debromination of three nona-BDEs in methanol using theoretical calculations, in which time-dependent density functional theory (TDDFT) combined with the polarizable continuum (PCM) model is applied. The selectivity of debromination was studied, and the major octa-BDE products photochemically debrominated from nona-BDEs were identified. We find that the debromination reaction results from the electronic transitions from π to σ* orbitals when nona-BDEs are exposed to UV-light in the sunlight region, at which point the two low-lying excited states for each nona-BDE are πσ*(5Br) and πσ*(4Br), which correlate to the σ* orbitals located on the penta-Br and tetra-Br substituted phenyls, respectively. Our calculations indicate that each nona-BDE may degrade to form three kinds of octa-BDE products via the πσ*(5Br) state, whereas only one kind of octa-BDEs can be formed via the πσ*(4Br) state. Our calculations can interpret the recent experiments successfully.