Understanding the pyrolysis mechanism of lignin is of great importance for improvement of thermochemical conversion processes and upgrading of biofuels. In this work, the pyrolysis of lignin model compound, guaiacol, at atmospheric pressure has been studied in a flow reactor by synchrotron vacuum ultraviolet photoionization mass spectrometry. Over 50 species were detected and identified in guaiacol pyrolysis, including alkanes, alkenes, cyclic alkenes, dienes, alkynes and aromatics. Major pathways proposed, including homolytic cleavage of O-CH3 bond, isomerization, dehydrogenation and hydrogenation, were evaluated using quantum chemical calculations. In the proposed pathways, homolytic cleavage of O-CH3 bond is regarded as an important route for the formation of catechol. H-atom and CH3-radical assisted demethoxylation mechanisms were proposed to interpret the formation of phenol and cresol, respectively. Furthermore, an energetically advantageous route including intramolecular H-immigration, ring-opening and decarbonylation was proposed to account for the formation of C-4 products. A series of aromatic products were detected in this work, which is helpful to understand the coking mechanism during pyrolysis of lignin.