Aiming at the problem of the irreversible deformation mechanism caused by thermal cycling in PBX-9502 （a type of TATB based polymer bonded explosive）， a three-phase microstructure model and an extended finite element method （XFEM） are used to establish a calculation model. The model takes into account the anisotropy of the TATB grains， the local differences in the thermodynamic properties of the bonding agent and its interface. Using the established model， the phenomenon and mechanism of irreversible deformation caused by thermal cycling of PBX-9502 are numerically analyzed. The results show that due to the severe anisotropy of TATB grains in PBX-9502 and the differences in the thermodynamic properties of TATB grains and binders， PBX-9502 specimens have internal deformation and stress concentration during thermal cycling. Resulting in the destruction of the bonding agent and the debonding of the interface and other internal damage to the material， which in turn led to the irreversible deformation of the PBX-9502 specimen under thermal cycling. At the end of the calculation， the axial strain of the PBX-9502 specimen reached 0.2%.