Abstract: Moving bed is widely used in industry, but the range of the velocity of cross-flow gas is limited by a phenomenon of “cavity”，which was investigated in a two-dimensional rectangular moving bed. The formation,developments of cavities in the moving bed were investigated and described with two particle materials and materials of wall. And the relationship between the size of a cavity and the velocity of cross-flow gas was investigated.A cavity could be created when the velocity of cross-flow gas or the pressure gradient was high enough, and the size of a cavity increased with the increase of cross-flow gas velocity.Based on force balance,a simplified nonlinear equation group with multiple physical parameters that affect the size of a cavity was developed and the cavity size vs cross-flow gas velocity relationship thus obtained agreed with experimental data.