Mg-50%Al4C3-6%Ce master alloy has been prepared by powder in-situ synthesis process. Phases compositions, micro-morphology and micro-area composition were analyzed by XRD (X-ray diffraction), SEM (scanning electron microscope) and EDS (energy dispersive spectrometer). The results show that the master alloy mainly contains lamellar Al4C3, rod-shaped Al4Ce and Mg. The microstructure of AZ91D alloy is obviously refined after Mg-50%Al4C3-6%Ce master alloy addition. The dendritic morphology of α-Mg evolves from a characteristic six-fold symmetrical shape to a petal-like shape. After adding 1.2wt% master alloy to AZ91D magnesium alloy, the average grain size is obviously decreased from 360 to 65 μm. The mechanism of grain refinement is ascribed to the Al4C3 particles acting as heterogeneous crystal nucleus of primary α-Mg and Ce can activate the potential Al4C3 particles on the solid-liquid interface