Contraction behavior of a liquid-solid fluidized bed has been investigated numerically.Based on a simple hydrodynamic model proposed by Brandani and Zhang (2006),a case study for solidparticles with a density of 3,000 kg/m3 and a diameter of 2.5×10-3 m is simulated in a two-dimensional fluidized bed (0.50 m height and 0.10 m width).Due to the continuity of numerical computation,there is a transition region between two zones of different solid holdups when the liquid velocity is suddenly changed.The top,middle and bottom interfaces are explored to obtain a reasonable interface height.The simulated results show that the steady time of the middle interface is more close to Gibilaro’s theory and suitable for describing the contraction process of a phase interface.Furthermore,the effect of liquid velocity and particle diameter is simulated in the other two-dimensional fluidized bed (0.10 m height and 0.02 m width) where the solid particles are glass beads whose properties are similar to those of the catalyst particles used in the alkylation process.The results also show good agreement with Gibilaro’s theory,and that larger particles lead to a more obvious bed contraction.