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Decabromodiphenyl ethane (DBDPE), as one of the important new brominated flame retardants, is widely utilized in a variety of plastic products. However, the pyrolysis mechanism of DBDPE remains uncertain. In this article, the evolution behavior of the main products during the thermal decomposition of DBDPE is investigated using density functional theory at the theoretical level of M06-2X/6-311++G(2df,p)//M06-2X/6-311+G(d). The results show that the initial reaction starts with the cleavage of the ethane bridge bond, with an absorbed heat value of 298 kJ/mol, and the cleavage of the C aromatic -Br bond generates bromine radical, which is the main competitive reaction, with a heat absorption of 317 kJ/mol. The initial degradation of DBDPE generates a large number of pentabromobenzyl radicals and bromine radicals, which facilitate the secondary pyrolysis of DBDPE to a certain extent, resulting in the formation of possible products such as pentabromobenzyl bromide, pentabromobenzene, pentabromotoluene, hexabromobenzene, pentabromostyrene, and hydrogen bromide. In the pyrolysis system of DBDPE with hydrogen radicals, the reactions are classified into two types: extraction reaction and addition reaction. It can be known that the addition reaction plays a dominant role in the degradation process, with a branching ratio of 89.8% at 1600 K. The degradation of DBDPE with hydrogen radicals is mainly characterized by debromination, and the main products are hydrogen bromide, low-brominated diphenyl ethanes, brominated phenanthrenes, and brominated monoaromatic compounds. In addition, the lowest reaction energy barrier (18 kJ/mol) is required for the addition of hydrogen radical to the ipso-C site of DBDPE. DBDPE is dangerous for the environment and humans since its fate includes bioaccumulation, biomagnification, and toxicity via hormones and endocrine disruptors., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)