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As one of the major components of lignocellulosic biomass, the current understanding on pyrolysis mechanism of hemicellulose is relatively limited compared to that of cellulose due to its random and complicated structure. Here, we reported an in-depth study on hemicellulose pyrolysis by combining advanced experimental techniques and quantum chemical calculation, using xylose, xylobiose and xylan as the model compounds. The structural evolution during pyrolysis was firstly characterized via in-situ diffuse reflectance infrared Fourier transform spectroscopy. The corresponding two-dimensional perturbation correlation infrared spectroscopy analysis shows that the dissociation of side chains dominates in the initial stage of xylan pyrolysis, followed by the cleavage of glycosidic bond and the opening of xylopyranose ring. The release behaviors of condensable vapors from hemicellulose pyrolysis obtained by thermogravimetric analyzer coupled with photoionization mass spectrometry indicate that the pyrolysis of xylan polymer with more side chains is prone to produce small compounds via ring rupture reactions compared to that of xylose and xylobiose. The possible reaction pathways of xylan pyrolysis were proposed based on the experimental results and were further confirmed via quantum chemical calculation. This work expands the size of model compounds from common monomers and dimers without side chains to larger tetramers with side chains. It was found that the dissociation of side chains and the cleavage of glycosidic bonds are competitive reactions. The backbone of xylan is decomposed following the dissociation of side chains to yield furfural, dianhydro xylose and other small compounds. [ABSTRACT FROM AUTHOR]