Graphene reinforced magnesium matrix composites have wide applications in automotive, electronics, aerospace and military fields due to the fascinating mechanical properties. However, it is difficult to realize the high strength and ductility simultaneously. In this work, the in situ liquid-state method was utilized to prepare GNPs/Mg6Zn composites via CO2/Mg chemical reaction. Tensile strength of the GNPs/Mg6Zn composites was improved with increasing content of the GNPs. Meantime, the composites also exhibit a notable plastic deformation stage, and especially the ductility of 0.12 GNPs/Mg6Zn composites reaches 27.6%. Therefore, this novel preparation method has great potential application for fabricating Mg matrix composites with high strength and high ductility.