Abstract: A self-made vibrating wavelike sloping plate device and a semi-continuous casting machine were combined. The semisolid billet of AZ91D magnesium alloy was prepared by the proposed process, and the effects of the different process conditions on the microstructures were investigated. The results showed that in a certain scope, the shearing and cooling time is enlarged with the increasing of the plate length, which leads to the grain refinement. Vibration can improve the nucleation rate and obviously refine the microstructure. At the lower casting speed, the nucleation rate in the crystallizer is higher, which causes the finer microstructure in the billet. Under the conditions that the casting temperature is 680°C, the sloping plate length is 400 mm and the casting speed is 100 mm/min, the semisolid billet of AZ91D magnesium alloy with fine microstructure is prepared by the semi-continuous casting process. After re-melting, the fine semisolid microstructure is obtained.