This study simplified a comprehensive multistep kinetic mechanism of fast biomass pyrolysis in a fluidized bed reactor (involving dense and dilute flows). A wide range of bioparticle sizes (0.2–2.0 mm) was investigated under four fluidization velocities (0.1–0.65 m/s). A multifluid model integrated with heterogeneous chemical reactions was developed to simulate fast biomass pyrolysis in a two-dimensional computational domain. The simplified multistep comprehensive reaction corresponded well to the product yield and composition data previously obtained through experimental pyrolysis. Most of the pyrolysis reactions occurred in the dense region and suddenly reduced the reaction rate in the freeboard. The relatively small size of the bioparticles facilitated their removal from the reactor, improved mixing performance, and resulted in a larger gas–bioparticle heat transfer coefficient and a faster reaction rate. However, particles that were excessively small (0.2 mm) shortened the bioparticle residence time substantially; this resulted in a considerable increase in the solid residue yield, the consumption of slightly more heat for pyrolysis, and further reduction in pyrolysis efficiency. Higher fluidization velocity led to shorter bioparticle residence time and improved the mixing performance of the solids, with a weak effect on the gas–bioparticle heat transfer coefficient. With a broader distribution region, this high velocity reduced the reaction rate slightly and increased the specific heat for pyrolysis substantially (from ∼879 kJ/kg to ∼4043 kJ/kg), thus reducing the pyrolysis efficiency (from 92.8% to 94.3% to 74.4%–76.9%).