Unimolecular decomposition mechanism is widely recognized as the major mechanism in lignin pyrolysis studies. In fact, during the real pyrolysis process, lignin and its derived intermediates, radicals and primary products are coexisting in the complex pyrolysis system. They may interact with each other, which will ultimately affect the whole pyrolysis process. However, current unimolecular decomposition mechanism does not take these interactions into consideration. In this study, density functional theory (DFT) methods and analytical pyrolysisgas chromatography/mass spectrometry (Py-GC/MS) experiments are employed to explore the intermolecular interaction mechanism of lignin pyrolysis. The results indicate that strong interactions between radicals and lignin or its derived pyrolytic products will take place during the pyrolysis process through H-abstraction reactions. Therefore, intermolecular interaction mechanism, rather than unimolecular decomposition mechanism, should dominate the lignin pyrolysis process. These interactions can promote the primary and secondary decomposition reactions of lignin and its derived pyrolytic products, and simultaneously stabilize the radicals.