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The conformational preferences of the lignin guaiacyl structural unit were studied by several quantitative chemistry calculation methods using vanillin as a model compound. The potential energy surfaces of the vanillin molecule were scanned by the methods of HF and DFT to find the most stable conformation, as well as three local minimum conformations and six transient conformations. Bonds strength of all kinds of bonds in vanillin molecules at five temperature were calculated by methods of DFT, MP2, and CBS. The calculation results indicated that temperature had little impact on bond strength; the large bond strength was Ar-OH, Ar-H, followed by Ar-CHO, Ar-OCH3, and the C-H in the aldehyde group, and O-CH3 bond strength in methoxyl was lowest (only 61 Kcal/mol), which may be cracked in pyrolysis. The calculation about the model dimer 1-α-β-O-4 also showed that the stable order was O-4 > 1-α> α-β> β-O, which agreed well with the fact that there are a lot of phenolic compounds in pyrolysis products of biomass or lignin.