The thermal degradation of decabromodiphenyl ether (BDE-209) was studied at the temperature range of 400~600℃. The types and contents of its degradation products were determined using GC and GC/MS techniques. The effects of temperature, time, and alkaline- and sulfur-containing compounds on the thermal degradation of BDE-209, as well as the formation of brominated dioxins, were discussed in this study. The results showed that both temperature and time were the two key factors for both the thermal degradation of BDE-209 and the formation of brominated dioxins. When BDE-209 was pyrolyzed at 450℃ for 40minutes, the brominated dioxins were generated to the maximum extent. The types of the products confirmed that alkaline compounds promoted the thermal degradation of BDE-209 by reaction with HBr formed from the BDE-209 debromination, and inhibited the formation of dioxins. Similarly, the sulfur compounds also promoted the degradation of BDE-209 and inhibited the formation of dioxins. However, the detailed inhibition mechanism needs to be further investigated in the future study. There are two kinds of degradation reactions for BDE-209, namely, debromination of phenyl rings and cleavage of ether bond. This research may provide a theoretical basis for the safe disposal of BDE-209-contained products and the reduction of secondary pollution.