Abstract: Pyrolysis experiments of 5-hydroxymethyl furfural (5-HMF) were conducted and the product composi-tion was detected by using a PY-GC-MS analyzer． The results show that nearly 86% of the pyrolysis product is fu-ran ring-contained substance，and that a certain amount of small molecules exist in the product，which means that the pyrolysis process of 5-HMF is mainly a side-chain degradation process concomitantly with the fracture of furan rings． Moreover，the converting law of the product in the pyrolysis process was supervised in real time via in-situ FT-IR． The results show that furfural and 5-methyl furfural mainly form when the temperature ranges from 130 to 230℃，furfuryl alcohol and 5-formyl-2-furfurylacetate mainly form after 250 ℃ and the formation continues to the middle and later periods of the whole process，while the dominant pyrolysis product of 5-HMF，namely furan-2，5-dicarbaldehyde，forms throughout the whole pyrolysis process． According to the results obtained from fast pyrolysis experiment and in-situ FT-IR，the pyrolysis mechanism is speculated as follows: as the most primary reaction，fu-ran-2，5-dicarbaldehyde forms from two consecutive dehydrogenations and then converts into 5-methyl furfural after the dehydroxylation and hydrogenation of hydroxymethyl; the whole fracture of hydroxymethyl side chain and alde-hyde group side chain leads to the formation of furfural and furfuryl alcohol; and 2-methylfuran may be produced via the secondary pyrolysis of furfural and furfuryl alcohol．

Key words: biomass, pyrolysis, 5-hydroxymethyl furfural, PY-GC-MS, in-situ FT-IR