Abstract: The semi-solid billet was first fabricated by decreasing pouring temperature,then remelted to semi-solid,and a AZ91D Mg alloy component for motor by high pressure casting process was at last formed.The microstructure and mechanical properties of the component were investigated by using optical microscopy,tensile tester and hardness tester.The results show that the semi-solid billet was composed of primary α-Mg phase,secondary α phase and γ-Mg17Al12 eutectic phase.Decreasing the pouring temperature could refine the microstructure.The remelted structure was composed of unmelted α-Mg solid solution and partial liquid transformation phase.After semi-solid die casting process,the microstructure was composed of α-Mg phase and net distributed eutectic phase.The effects of microstructure defects and casting internal defects on mechanical properties of the component prepared by the process were remarkable,therefore the semi-solid die casting process needed to be optimized further.