In order to further understand the pyrolysis mechanism of decabromodiphenyl ethane (DBDPE) and the evolution process of its main products, the thermal degradation reaction mechanism of DBDPE was studied using density functional theory (DFT) method M06-2X/6-311G(d,p). Possible reaction paths for DBDPE pyrolysis were designed, and the kinetic and thermodynamic parameters of various reaction pathways were calculated. The results show that the initial pyrolysis of DBDPE is dominated by the fracture of aliphatic H₂C—CH₂ bond, which results in the formation of a large number of pentabromobenzyl radicals. The bromine radical produced by C_aromatic—Br bond fracture is a competitive reaction channel, and the bromine radical can further react with the pentabromobenzyl radical to form pentabromobenzyl bromide. The pentabromobenzyl radicals and bromine radicals produced in the initial reaction can promote the decomposition of DBDPE, which are mainly reflected in the abstraction of H atoms from aliphatic H₂C—CH₂ bonds with energy barriers of 21.9 kJ/mol and 38.3 kJ/mol, respectively, resulting in the formation of pentabromotoluene, pentabromostyrene, hexabromobenzene, and hydrogen bromide. In the pyrolysis reaction processes of DBDPE with hydrogen radical, the addition of hydrogen radical reduces the reaction energy barrier of DBDPE pyrolysis, and the reaction energy barrier of adding hydrogen radical to the aromatic-carbon of C_aromatic—CH₂ of DBDPE is the lowest (17.9 kJ/mol). The main products of co-pyrolysis of DBDPE with H radicals are hydrogen bromide, polybrominated diphenylethane, and brominated monoaromatic compounds.