Abstract: The circulating fluidized-bed gasification is a potential industrial technology for high efficient utilization of biomass in middle- and large-scaled power stations. A fast internally circulating fluidized-bed steam gasification technology has developed to obtain a high-grade synthesis gas without using pure O2. Some demonstration and/or commercial plants supported by European Union and its members are in underway. Computational fluid dynamics (CFD), an alternative method to the step-by-step experimental scale-up, is employed to simulate a laboratory-sized cold set-up in this paper. A modified particle-bed model has been attempted to investigate the fluid dynamic stability of gas-solid system in the bubbling fluidized bed. Superficial gas velocity is one of vital factors to influence the dynamic flow behaviour and pressure fluctuation. The bubble number and the size of the bubble increase with an increase of superficial gas velocity. Pressure fluctuation becomes higher with increasing inlet gas velocity. The higher gas velocity is and the greater time-averaged pressure drop is. Solid circulation is a single-cell pattern in the fast internally circulating fluidized bed. These results appear in good agreement with the experiments, which will be helpful for scaling up and designing the fast internally circulating fluidized-bed biomass gasifier.