Comprehension in hemicellulose pyrolysis is critical to generate renewable fuel and valuable chemical. Herein, a self-designed tubular reactor was applied to observe the appearance alteration and chemical structure evolution during the whole xylan pyrolysis process. Before 200 degrees C, it was free moisture removal stage without significant chemical structure alteration. Xylan began to depolymerize at 200 degrees C corresponding with the appearance change from its original state to dark brown, cleavage of branched-chain and primary product acids & ketones generation. The main chain of xylan was completely broken at 250-350 degrees C via beta-1,4-glycosidic bond cleavage, dehydration, decarboxylation, and decarbonylation reaction. Acids were mainly originated from hemicellulose pyrolysis. The typical signals from FTIR,C-13 CP/MAS NMR were disappeared at 350 degrees C. In the carbonation stage, the C/H and C/O ratio reached 2.01 and 4.54, leading to the aromaticity enhancement of char and formation of carbon-centered radicals.