The conventional liquid die casting and rheo-diecasting processes of magnesium matrix composites were studied with the numerical simulation method. The constitutive model of semi-solid Mg2Si/AM60 composite had been established in our prior study. The pressure and the inside defects of the material during rheo-diecasting is realized. The comparison of two forming processes is done. The results indicate that the rheo-diecasting won't lead to interior defects such as gas-hole etc. The simulation results are in accord with the experimental ones.