The invention discloses a graphene and carbon nanotube mixed enhanced metal-matrix composite material and a preparation method thereof. The graphene and carbon nanotube mixed enhanced metal-matrix composite material is characterized in that graphene and a carbon nanotube are mutually connected to constitute an enhanced network in a metal matrix, wherein the graphene is few-layer graphene with 10 layers or less, the radius-thickness ratio of the graphene is larger than 200, and the volume fraction of the graphene is 0.1-1%; and the carbon nanotube is a single-wall, double-wall or multi-wall carbon nanotube, the length-diameter ratio of the carbon nanotube is larger than 20, and the volume fraction of the carbon nanotube is 0.5-5%. Compared with the composite material enhanced only by the carbon nanotube, the graphene and carbon nanotube mixed enhanced metal-matrix composite material disclosed by the invention not only has greatly improved mechanical properties, but also has more excellent electric conduction and heat conduction properties, and is a multi-purpose structure and function integrated material. In addition, the preparation method provided by the invention based on slurry blending and graphene oxide reduction is simple and efficient and is easy for large-scale production.