The industrial application of magnesium alloys is limited by their low strength and poor plasticity. The additionof grain refiners to magnesium alloys can improve their microstructure, strength and plasticity, which is one of the mosteffective methods for improving their properties. In this paper, a refiner containing GNPs (graphene nanosheets) wasprepared by ball milling and powder metallurgy. Raman spectroscopy was used to preliminarily explore the influenceof different ball milling parameters on the quality of the GNPs, and the relative optimal ball milling parameters were300 r/min and 2 h, which ensured the high quality of the GNPs after ball milling. The composite powder was cold pressedand sintered at 400 ℃×2 h to obtain a well-formed Al-2.5GNPs refiner containing a GNP phase with a dense structure. Thebinding situation of the mixed powder and the refinement and strengthening mechanism of Al4C3 and the GNPs in theAZ31 magnesium alloy were preliminarily analysed using OM, SEM, XRD and TEM characterization methods. Byadding a 1.5 wt. % Al-2.5GNPs refiner at 720 ℃, the grain size of the AZ31 magnesium alloy decreases from 228.8 μm to65.6 μm, the tensile strength increases from 112.6 MPa to 178.2 MPa, and the elongation increases from 8.9% to 16.7%,increasing by 58.3% and 87.6%, respectively.