The addition of effective nucleating particles in the melt to achieve grain refinement has become the most widely used method for the casting industries. In this study, a novel GNP@MgO particle with a nanocomposite structure was prepared by utilizing an in-situ reaction of the carbon source gas with Mg melt. The results showed that the particles can significantly reduce the average grain size of Mg-9Al alloy from 130.4 µm to 13.1 µm, and achieve an ultra-high grain refinement efficiency of 90%. The refinement mechanisms are that the Al₄C₃ phase can act as a heterogeneous nucleation site for α-Mg grains due to the orientation relationship as (001)_Al4C3//(002)_Mg. Meanwhile, the particle distribution model shows that the velocity of MgO particles is much higher than the growth rate of α-Mg grains. Therefore, it is pushed to the vicinity of grain boundaries during solidification, effectively limiting the growth of α-Mg grains. The remarkable grain refinement effect was achieved through the synergistic modulation of Al₄C₃ and MgO particles. This work may provide new insight into designing high efficiency grain refiners for Mg-Al alloys.