To study the detailed aging mechanisms of polyethylene glycol (PEG) adhesive, based on the B3LYP method of density functional theory (DFT) and canonical variational transition state theory combined with a small-curvature tunneling correction (CVT/SCT), the molecular simulation and calculation for the aging reaction type of PEG molecule in the presencee of PEG unimolecule and NO₂ molecule were carried out. Results show that there are two kinds of degradation breaking of C—O bond breaking reaction and hydrogen atom transfer reaction under the PEG unimolecular reaction condition. The bond dissociation energy of C(2)—O(2) bond is smaller than those of the chemical bonds O(1)—C(1) and C(1)—C(2), the C(2)—O(2) breaking is the trigger point of the cleavage reaction of PEG unimolecular. The hydrogen atom transfer reaction is the process of gradual transfer of hydrogen atom connected to C(1) to O(2) atom and the amount of heat absorbed by the reaction is 29.19 kJ·mol^-1. Under the participation of NO₂ molecule, there are three kinds of aging reaction modes, namely the degradation breaking reactions of PEG molecule, nitration reaction and cyclization reaction, the activation energies required is less than that of the aging reactions of PEG unimolecular.The change range of bond dissociation energies and activation energies required by aging reactions of PEG molecule with the polymerization degree of 2, 4, 6, 8 and 10 are less than 3 kJ·mol^-1, indicating that the polymerization degree has little effect on the reaction. At the storage temperature of 20-60 ℃, the rate constants of degradation breaking reactions and cyclization reaction under the participation of NO₂ molecule are 10³-10²⁸ times as much as that of hydrogen atom transfer reaction. The basic aging rule of PEG molecule in the presence of NO₂ molecule is obtained, considering that the cleavage reaction, nitration reaction and cyclization reaction for PEG under the participation of NO₂ molecule are the main aging modes, the hydrogen transfer reaction of PEG unimolecular are the secondary aging mode, while the occurrence of the C—O bond breaking reaction is the most difficult.