Gas distribution patterns exert a great impact on the gas-solid flow, heat and mass transfer, and reaction characteristics and thus affect significantly the thermal conversion efficiency inside the biomass gasifier. In this study, a twodimensional biomass gasification model for a bubbling spout fluidized-bed gasifier (BSFBG) is established based on the kinetic theory of granular mixture (KTGM), and the prediction accuracy of the model is verified through a comparison between simulation values and experimental values. Furthermore, with the total mass flow rate of the inlet gas set constant, the study has investigated how the ratio of the inlet cross-sectional area to reactor area (A(m)/A) and the auxiliary gas velocity (U-f) affect the gas-solid flow, heat transfer, and gasification performance in the BSFBG. As the results indicate, with A(m)/A increasing, the local particle flow structure becomes increasingly complex, particle volume fraction and temperature get more evenly distributed, and the volume fraction of outlet key gas species (CO, H-2, and CH4) and the lower heating value (LHV) is gradually rising. Besides, compared with the even gas distribution condition (A(m)/A = 100%), the formation of overall and local counterclockwise particle circulation is facilitated by either a relatively high U-f with a low A(m)/A (<= 50%) or a relatively low U-f with a high A(m)/A (>= 75%). The circulation structure strengthens the gas-solid mixing in the pyrolysis zone and gasification zone in the middle-upper part of the dense phase bed, which makes the volume fraction of outlet key gas species and LHV increase significantly. In the meanwhile, when A(m)/A = 75% and U-f = 1.0U mf , both the volume fraction and LHV reach the maximums, indicating that the overall gasification performance achieves the optimal state. In conclusion, this study is of reference value for the investigation and intensification of the gasification process, which helps to optimize BSFBG design and thus improve the biomass thermal conversion efficiency.