Polybrominated diphenyl ethers (PBDEs) have received special environmental concern because of their potential toxicity to humans and wildlife worldwide. However, their photochemical degradation mechanisms remain largely unknown. Herein, a PCM/TD-DFT scheme (time-dependent density functional theory combined with the polarizable continuum model) augmented with explicit solute-solvent interactions is used to explore the promotive effects of the toluene solvent on the photochemical degradation debromination of deca-BDE (BDE209). The face-to-face pi-pi interactions between penta-bromine-substituted phenyl and toluene are investigated. The calculations indicate that the face-to-face pi-pi interaction plays an important role in the low-lying pi ->sigma* transitions of BDE209-toluene pi-stacking complex at around 300 nm in the sunlight region, which leads to notable changes for the pi sigma* excited states and which promotes the breaking of the C-Br bonds. The photodegradation reaction via an intermolecular charge-transfer excited state formed by the electronic transition from a pi orbital of toluene to a sigma* orbital of BDE209 is found to be a dominant mechanism. Our calculation results reveal the mechanism of how the participation of an explicit toluene solvent molecule catalyzes the photodegradation of BDE209 and explain the experimental results successfully. The present study may provide helpful information for the removal of PBDE contamination.