Cellular automaton （CA） is applied to simulating the microstructure evolution during the solidification， and finite element method （FEM） was used for solving the conservation equations which govern the macro-transport phenomena during the twin-roll casting process of the magnesium strip in the molten pool. Coupling CA with FEM can predict the microstructure evolution during solidification involving nucleation and growth of the grains. On the basis of this CA-FEM method，the effects of the main parameters involving pouring temperature， casting speed and pool height on the characteristic of solidification microstructure such as grain size and orientation can be quantitatively clarified by numerical simulation， which can provide the theoretical basis for the optimization of the processing parameters and the control of the solidification structure.