Abstract: The semi-solid preparation process optimization of magnesium alloy had been studied by positive alternation method with rheo-diecasting process and twin-screw in high shear rate.The results shows that the solid fraction of semi-solid slurry and the round degree of primary α-phase are intensively influenced by barrel temperature of screw mixer,and the primary grain size is mainly controlled by shear rate of twin-screw.The pertinent processing parameters are summarized as follows:pouring temperature of the Mg melt is 605 ℃;barrel preheating temperature is 575 ℃ and shear rate is 6 830 s-1.The tensile strength,elongation and hardness of semi-solid AZ91D alloy are 216 MPa,2.3％ and 74 HB.