Pyrolysis of lignocellulosic biomass is a sustainable and low-cost technology for the production of bio-oil and valued-added chemicals. The present study aims to develop a fundamental and reasonable mechanistic model for hemicellulose pyrolysis. Thermogravimetric analysis-Fourier transform infrared spectroscopy and pyrolysis-gas chromatography/mass spectrometry were performed to investigate the formation mechanisms of the main products, and the density functional theory study was used to find the most rational pathway. A detailed mechanistic model for hemicellulose pyrolysis was developed by using O-acetyl-4-O-methylglucuronoxylan as the model compound. As a result, the energy barriers of the formation of anhydrosugars, low molecular weight products, gas, and furfural were obtained. The study is able to explain the fast pyrolysis behavior and provide a basis for developing models that can predict the yields of products formed in different conditions.