D microstructure evolution simulation of Mg die casting alloy was conducted by CA（Cellular Automaton） model. Continuous nucleation model was used to describe heterogeneous nucleation of the liquid metal. Gauss distribution function was employed to reveal the relation between the density of nucleation sites and the temperature. At a given undercooling

the nuclei density can be done by intergrating the function. Dendritic tip growth kinetics

effects of temperature field and crystal preferential growth direction < 1 0 1 0> were taken into account in grain growth model. The macro temperature field coupled with microstructure calculation was made. The simulated results were verified by standard tensile bar of Mg die casting alloy. The simulated results were well in agreement with the experiment on average grain sizes.