Abstract: At high temperatures or under sunlight, Deca-BDEs can be transformed and depredated by microbial action into low brominated diphenyl ethers, which are more toxic. Since BDE209is the main component of Deca-BDEs, it is very important to study the environmental degradation behaviors of BDE209to determine the source of lower brominated diphenyl ethers and thus eliminate the environmental pollution. This study moves forward from the degradation mechanism to analyzing the degradation products in different lights and at different degradation stages. Moreover, in order to explore faster and more effective disposal technology, the present study carried out laboratory simulation tests and comparative analysis tests for related factors such as irradiation intensity, different organic solvents and solid medias that influence the degradation behaviors of BDE209. The results showed that, the degradation rate as well as the degradation degree of solar are slower than that of UV light; the degradation products are also different for the solar and UV lights. In the environment, photodegradation, among the aforementioned mechanisms, is the primary pathway for Deca-BDEs to be degraded. This study revealed that the photodegradation reaction is a first order reaction, and debromination occures step-by-step with the final degradation productions containing much low bromine generation homologue. However, a series of potential degradation steps during the photolysis reaction and whole photodegradation mechanism remain to be investigated. Meanwhile, the study of Deca-BDEs calls for a clear description of photodegradation mechanisms, the influencing factors, and the degradation pathway. The studies on microbial degradation mainly focus on the degradation characteristics of specific microorganism categories such as bacteria and amylase (including degradation rate and degradation effect), but the degradation pathway and transformation of Deca-BDEs as well as the degradation product identification are not entirely clear. In addition, the mechanism of Deca-BDEs transformation into brominated dioxins (PBDD/Fs) at high temperature is not thoroughly analyzed. Due to the high hydrophobicity of BDE209, it mainly exists in the solid particles in the atmosphere. Therefore, future research should focus more on the photodegradation behavior of BDE209in gas eous and solid phases. More attention should also be paid to microdegradation as the remediation technology for sites contaminated with Deca-BDEs, as well as the degradation path and influencing factors of Deca-BDEs during the process of dismantling electronic waste.