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This study investigated the changes in biomass physicochemical characteristics during torrefaction and its influence on the resulting pyrolysis behavior. Torrefaction reduced biomass hemicellulose content and increased the high heating value and the mass energy density. Two-dimensional perturbation correlation analysis, based on diffuse reflectance infrared Fourier transform spectroscopy, showed that the main reactions occurring during torrefaction were dehydration, deacetylation, and cleavage of ether linkages. A distributed activation energy model with three Gaussian functions and weighting factors was used to study the pyrolysis kinetics, and it was found that as the torrefaction temperature increased, the contribution of lower activation energy parallel reactions to devolatilization decreased, while condensation became more important. The yields of acids and furans from the pyrolysis of torrefied biomass also decreased. The lignin side branches were cleaved during high temperature torrefaction, resulting in lower yields of phenols with side branches and increased production of phenols without side branches.