Pyrolysis of lignocellulose biomass to produce various fuels and chemicals has gained increasing interest in recent decades. An in-depth understanding of the biomass pyrolysis reaction mechanisms is essential for the advancement of pyrolysis techniques. Quantum chemistry (QC) modeling is a powerful approach for the pyrolysis mechanism investigation at the atomic/molecular level. Despite a short history of only about 2 decades, its application to the biomass pyrolysis mechanism exploration has been well-developed, along with the fast advances of supercomputer and computational codes in the new century. This review addresses the recent progress on the pyrolysis mechanism of the three basic biomass components (cellulose, hemicellulose, and lignin) by QC modeling. On the basis of the QC modeling results reported in the literature, the current review critically summarizes the key developments about the pyrolysis chemistry of biomass by focusing on their microscopic elementary reactions, the formation routes of typical products, bimolecular interactions within or between biomass components, and catalytic effects of various catalysts. Notably, there are great gaps between the theoretical models employed in QC modeling and the natural biomass substance in the pyrolysis process. Therefore, a brief analysis of the challenges and future research perspectives is provided for the biomass pyrolysis mechanism research.