Based on the microstructure evolution of AZ31 magnesium alloy during hot compression, a recrystallization grain size model and dynamic recrystallization percentage model were established by the cellular automaton model. The dislocation density model, critical dislocation density model, nucleation rate model and grain growth model of magnesium alloy AZ31 were deduced by thermal compression experiments under different deformation conditions. Combining the specific evolution rules of cellular automata, cellular automata model was established. The simulation results and accury of cellular automata were verified by stress-strain curves and grain size. Based on the experimental data and the JMAK theory, the recrystallization grain size model and dynamic recrystallization percentage model were deduced. With DEFORM-3D analysis software, the change of grain size distribution and dynamic recrystallization percentage distribution in deformation were obtained.