Abstract: In order to understand the pyrolysis mechanism of lignin, 3-hydroxy-1-(4-hydroxyphenyl)-2-phenoxypropan-1-one with Cα carbonyl group was selected as a β-O-4 type lignin model dimer, and its pyrolysis process was theoretically calculated by density functional theory methods at B3LYP level with 6-31++G(d, p) basis set.The calculation results indicated that the carbonyl substituent on the Cα dramatically could significantly decrease the bond dissociation energy (EBD) of Cβ-O, while the bond dissociation energy of Cα-Cβ was augmented.The bond dissociation energy of Cβ-O was 91.5 kJ/mol lower than that of Cα-Cβ.Thus, it could be deduced that the dimer was mainly decomposed through the homolytic cleavage of the Cβ-O linkage, and major phenolic pyrolytic products of phenol and 4-hydroxybenzaldehyde, minor parts of 4-hydroxyacetophenone were formed in this process. The R7-a kinetic way was considered as the optimum way to form 4-hydroxybenzaldehyde, and the energy barrier was 236.6 kJ/mol.