In this paper, a simplified cellular automaton (CA) model was proposed for modeling the evolution of microstructure in solidification process of AZ91D magnesium alloy. Since the calculation time was significantly reduced, it might be used to predict the microstructure field of a real Mg component after solidification. The stochastic nucleation, competitive growth processes of many grains with various crystallographic orientations and the formation of eutectic structure were also taken into account. Furthermore, step castings were poured with sand and permanent molds and metallographic experiments were carried out for validating the developed models. It was shown that the simulation results are in good agreement with those obtained in the experiments.