Abstract: The contracting and expanding processes of liquidsolid flow have been analyzed theoretically and simulated numerically in a twodimensional fluidized bed using the commercial Computational Fluid Dynamics (CFD) code, CFX4.4 with water and glass beads as the liquid and solid phases. A liquidsolid bed in equilibrium would transfer to a new equilibrium state after the liquid flux changes greatly. When the liquid flux is suddenly decreased, the bed is divided distinctly into two zones: an upper zone and a lower zone. The liquid volume fraction keeps the original value at the upper zone, whilst the liquid volume fraction becomes a new value corresponding to the new liquid flux. In fact, there exists a narrow region between the above two zones, which can be regarded as the “internal interface”. Bed surface and internal interface change linearly with the simulated time although the bed surface and internal interface moved downwards and upwards, respectively. When the liquid flux is suddenly increased, both the bed surface and internal interface rising with time are in nonlinear form, which is attributed to an influence of the gravitational instability. However, the effect of the simulated period of the expansion process on the response time of the contracting process can be neglected. The idealized expansion response time is close to the contraction response time, which is far shorter than the simulated value. All simulated data are in fair agreement with the theoretic and experimental data reported by other researchers.