Numerical simulation of gas-solid two-phase flows in circulating fluidized bed is proved to be a low-cost and high-efficient method to research the instability essence of flow character. In this work, numerical simulation based on the discrete element (DEM) method is applied to analyze behaviors in a rectangular cross-section fluidized bed. The models of physical and mathematical models are introduced in detail, and the schematic of DEM method also described clearly. It indicates that the DEM method may be used as a powerful tool for the simulation of the gas-solid flow.