In this work, the reaction pathways of hemicellulose were investigated for a complete understanding of the mechanism of biomass pyrolysis in thermal plasma. Energies of the possible reaction pathways of three model compounds of hemicellulose, xylose, O-acetyl xylose and 4-O-MeGlcA, in hydrogen plasma were estimated through density functional theory (DFT), and the most favorable reaction routes were proposed. Combining with our previous work, the effects of the biomass composition on their pyrolysis behavior in hydrogen plasma were discussed, and the production distribution was predicted. The reaction mechanism of biomass pyrolysis in hydrogen plasma was clarified finally, which indicated that active FP in plasma played a very important role in dehydrogenation reactions. (C) 2016 Elsevier Ltd. All rights reserved.