In order to improve the mechanical properties of the metal matrix composite, the copper-graphene composites were prepared by FSP (friction stir processing) method. The samples were analyzed through the metal microstructure observation test and mechanical test, the effects of agitation speed and the amount of graphite on the microstructure and tensile strength of the composites were investigated, and the strengthening mechanism of the composites was studied. The results show that the effect of graphene on copper matrix is mainly reflected in load transfer and hindering dislocation movement and grain boundary growth in copper matrix. With the introduction of graphene, the grain in the nugget area has been significantly refined. The reason for grain refinement is the mechanical stirring of the stirring head and the obstruction of graphene to grain growth during recrystallization. The tensile strength of the copper-graphene composites is 5% higher than that of the base metal, up to 27749 MPa. The copper-graphene composites with good properties and uniform distribution of graphene can be prepared by FSP method, which can effectively improve the mechanical properties of copper alloy and provide theoretical basis and technical reference for its wide application.