Abstract: Different from the traditional unimolecular decomposition mechanism, focusing on the bimolecular reaction pyrolysis mechanism, the α—O—4 type lignin dimer model compound benzyl phenyl ether（BPE） is selected as the research object. The density functional theory（DFT） GGA/RPBE method and DNP basis set are used to perform geometric optimization and frequency calculations on the reactants, intermediates, transition states, and products during the pyrolysis process. The study focuses on the impact of substituents in lignin on the intermolecular interaction relationships. The results show that substituents will increase the activity of each chemical bond in unimolecular reactions, for example, CH₃ substituents will significantly promote the homolytic cleavage of C—O bonds. For bimolecular reactions, although intense interactions exist between lignin molecules and free radicals, it is proved that the presence of substituents can inhibit the molecular interaction relationship. Furthermore, the H radicals on the substituents display weaker reactivity compared to those on the lignin—linked bonds. The O—C_aromatic homolytic cleavage dominates the primary reaction pathway for dehydrodimer radicals, rather than the C_α—O bond.