A three-dimensional cellular automata (3D-CA) model was established to visualize and quantitatively predict the microstructural evolution of AZ31 magnesium alloy during hot deformation by thermal compression test and electronic backscatter characterization (EBSD). According to the true stress-strain curve obtained from the test, the values of the parameters of the 3D-CA model under the test conditions are determined, and the relationship between the parameters of the model and the deformation conditions (strain, deformation temperature and strain rate) is established. The flow behavior and microstructure evolution of AZ31 magnesium alloy during hot deformation were simulated and discussed by using the established 3D-CA model. The results show that the recrystallization integral increases with the increase of strain, and increases with the increase of deformation temperature or the decrease of strain rate. Raising strain rate or decreasing temperature can refine recrystallized grains. The simulation results are in good agreement with the experimental results. The relative error is between 4.5% and 16.2%. The 3D-CA model can accurately predict the microstructure evolution of magnesium alloy AZ31.