Abstract: Three-dimensionally coupled computational fluid dynamics (CFD) and discrete element method (DEM) are used to investigate the gas–solid hydrodynamics and mixing characteristics of a three-dimensional rectangular bubbling fluidized bed. The numerical study is conducted referring to the quasi-3D experimental apparatus investigated by Goldschmidt et al. [1]. The simulation results turn out that time-averaged solid volume fraction and instantaneous particle height agree well with the experimental data. To investigate the solid behavior, the solid velocity and flux distribution are extracted and the influence of the gas velocity on the solid velocity is discussed. It is found that higher gas velocity induces the formation of double-recirculation pattern in which solid phase mainly moves from the near wall region to the central region in the lower bed and oppositely in the upper bed. At the same time, the solid mixing is faster under higher inlet velocity, and that particles in vertical directions tend to mix better than those in lateral directions. At last, we find the particle dispersion coefficient is very anisotropic in three normal directions. In accordance with the results of most experiments, the geometry structure strongly influenced the coefficient value. We also find when inlet gas velocity is higher, the dispersion coefficient increases, for the particle velocities are higher due to greater momentum exchange.