Abstract: The mechanism of 5-hydroxymethylfurfural(5-HMF) pyrolysis was studied by density functional theory B3LYP/6-31G++(d, p). The results show that the energy barrier of generating furfural by hydroxy side chain removal of 5-HMF is 322.8 kJ/mol, and the energy barrier of 2-furfuryl alcohol by aldehyde group side chain removal of 5-HMF is 375.4 kJ/mol. It is indicated that the appearance of furfuryl is earlier than 2-furfural in the pyrolysis process of 5-HMF. Further pyrolysis of furfural can occur the ring-opening reaction to form benzene ring with the reaction energy barrier of 370.8 kJ/mol, which explains that the furan ring can undergo deoxygenation and cyclization reaction. 5-HMF can directly generate ring-opening reaction in the case of H₂O participation or without H₂O participation to obtain enol compounds or enone compounds. The energy barrier of ring-opening reaction with H₂O participation is 287.6 kJ/mol, and the energy barrier of ring-opening reaction without H₂O participation is 279.1 kJ/mol. Thus, the participation of water molecules is not conducive to the ring opening of 5-HMF.