Abstract: In order to understand the pyrolysis mechanism of hemicellulose and to identify the formation pathways of key products during pyrolysis, the pyrolysis processes of O-acetyl-xylopyranose are investigated by using density functional theory methods at B3LYP/6-31G++(d,p) level. In the pyrolysis, O-acetyl-xylopyranose firstly decomposes to form acetic acid and IM₁ with an energy barrier of 269.4 kJ/mol, and then IM₁ is converted to acyclic carbonyl isomer IM₂ with a low energy barrier of 181.8 kJ/mol. IM₂ further decomposes to form all sorts of small molecules through four possible pyrolytic reaction pathways. The equilibrium geometries of the reactants, transition states, intermediate and products were fully optimized, and the standard thermodynamic and kinetic parameters of every reaction pathway were calculated. The calculation results show that reaction pathways (2) and (4) are the major reaction channels in pyrolysis of O-acetyl-xylopyranose and the major products are low molecular products such as acetic acid, acetaldehyde, glycolaldehyde, acetone, CO, CO₂ and CH₄, which is according with related analysis of experimental results.