Abstract: Thermal degradation mechanism of polybutylene terephthalate (PBT) dimer was studied by density functional theory (DFT) method M06-2X/6-311G(d). Eight possible reaction paths were designed for the thermal decomposition of PBT dimer, and the thermodynamic and kinetic parameters of elementary reaction steps in each reaction path were calculated. Calculation results show that, in the initial pyrolysis process of PBT, the energy barrier of concerted reaction occurring on the main chain is significantly lower than that of the radical reaction, so terephthalic acid, monobutenyl terephthalate, dibutenyl terephthalate and diterephthalate-1,4-butadiester formed by concerted reaction are main products in PBT initial pyrolysis. The energy barrier of the main chain fracture through the six-membered cyclic transition state is lower than that through the four-membered cyclic transition state, and the main chain fracture of PBT is mainly through the concerted reaction with six-membered cyclic transition state. In addition, the secondary degradation reaction of the main products of PBT pyrolysis was also discussed. It is found that the main products such as 1,3-butadiene, tetrahydrofuran, benzene, CO₂ and benzoic acid are mainly generated through concerted reaction in the processes of secondary degradation.