A β-1-type lignin dimer, 1,2-bis(3,5-dimethoxyphenyl)propane-1,3-diol was employed as a model compound in this study. The pyrolysis mechanisms and formation pathways of the pyrolytic products were investigated by using density functional theory (DFT) calculations and analytical pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). Four possible initial pyrolysis mechanisms were proposed, including the Cα-Cβ homolysis mechanism and three concerted decomposition mechanisms (1, 2, and 3). Results indicated that the lignin dimer decomposed via two concerted decomposition mechanisms, forming 3,5-dimethoxybenzaldehyde, 1,3-dimethoxy-5-vinylbenzene, 3-hydroxy-5-methoxybenzaldehyde, and 3-methoxybenzaldehyde. 3,5-Dimethoxybenzaldehyde was the major product, accounting for greater than 50% of all pyrolytic products. In addition to the two concerted decomposition mechanisms, Cα-Cβ homolysis was a secondary pyrolysis mechanism during the lignin dimer pyrolysis process, and the pyrolytic products included 3,5-dimethoxybenzyl alcohol, 3,5-dimethoxyphenethyl alcohol, 1,3-dimethoxybenzene, and 1,3-dimethoxy-5-methylbenzene. A third concerted decomposition mechanism was judged to be the least likely pathway to occur because of the high activation energy requirement.