Biomass tar as a byproduct during the gasification process can lead to many problems, such as condensation and subsequent plugging downstream. However, the knowledge of the correlations between the tar formation and conversion and the gas-solid hydrodynamics in fluidized bed gasifiers (FBGs) is still limited compared to other aspects of biomass gasification. In this study, a three-dimensional comprehensive model that simultaneously coupled the detailed tar kinetics and the homo- and heterogeneous kinetics with the multi-phase particle-in-cell hydrodynamics was constructed for a biomass FBG. Numerical predictions were in very good agreement with experimental data. The influences of the oxygen concentration in the gasifying agent and the biomass particle size distributions on the gasification process were investigated using the comprehensive model. Numerical findings indicated that a high oxygen concentration of the gasifying agent reduces the nitrogen dilute effects and increases the gasification temperature, giving rise to a low tar yield and high gasification efficiency. A small particle size enhances the releasing and diffusing rates of volatiles from biomass particles, resulting in a low tar yield and high gas yield and gasification efficiency. It is believed that the comprehensive model can assist to explore the tar formation and conversion behaviors on the particle scale for the biomass gasification process.