2D simulations fail to precisely consider the 3D effects of boundary conditions imposed by the column wall and to accurately capture the complex inherent gas-solids flow, so 2D simulations cannot completely take the place of 3D simulations and can only be used for qualitative studies. Recently, Li et al. [3] proposed a novel pseudo two-dimensional (2.5D) simulation approach with the benefits of 2D Cartesian and axisymmetric assumptions to simulate the gas-solids flow in a lab-scale bubbling fluidized bed. In this work, the novel 2.5D simulation approach was applied to simulate the gas-solids flow in a cylindrical CFB riser in the open source code MFIX. In the 2.5D simulation approach, the ratio between the plate half width and the wedge radius is an important parameter determining the transition from 2D Cartesian assumption to axisymmetric flow assumption. The present work comprehensively investigated the effects of this parameter on the hydrodynamics of CFB riser. The simulation results showed that this parameter significantly affected the predictions of axial and radial distributions of mean solids holdup, vertical solids velocity and solids flux, and a suitable value was obtained for the qualitative and quantitative study of the complex hydrodynamics of the CFB riser. Furthermore, by comparing the 2.5D and 2D simulation results against experimental data, it has been clearly demonstrated that the 2.5D simulation approach gives better agreement with experimental observations. For the same fast fluidization system, when Gs is constant, increasing superficial gas velocity from 5 m/s to 7 m/s, the suitable L/R value decreased from 0.53 to 0.32. (C) 2015 Elsevier B.V. All rights reserved.