In this study, we present a computational fluid dynamics/discrete element method (CFD-DEM) framework for biomass fast pyrolysis. With model validations against experiments, the effects of the particle shrinkage and superficial gas velocity on the pyrolysis process in the fluidized bed reactor are investigated. The results show that the shrinkage pattern has minor effect on the product yields, but it greatly affects the entrainment behaviors. It is found that the constant density shrinkage pattern leads to the longest residence time, and the constant size shrinkage pattern results in the shortest residence time. Interestingly, the time that biomass particles stay in the dense region shows a reverse trend. It is demonstrated that the gas velocity has a minor effect on the heat-transfer rate and chemical conversion rate of biomass particles, but it has a profound impact on the distributions of solid concentration and heterogeneous reactions.