The invention provides a high-thermal-conductivity metal-based composite material with a graphene-modified interface and a preparation method of the high-thermal-conductivity metal-based composite material. The high-thermal-conductivity metal-based composite material with the graphene-modified interface comprises diamond and graphene-modified metal powder. The preparation method comprises the following steps that 1, annealing reduction is carried out on the metal powder so as to remove oxides on the surface; 2, the metal powder subjected to annealing reduction is coated with a solid carbon source or a gas carbon source, and high-temperature in-situ growth is carried out under the protection of hydrogen atmosphere to obtain graphene-coated metal powder; and 3, the graphene-coated metal powder is mixed with the diamond, and the high-thermal-conductivity metal-based composite material with the graphene-modified interface is prepared through hot-pressing sintering. According to the preparation method, the interface wettability between the metal matrix and the diamond particles is effectively improved, and the thermal resistance of the interface is reduced; and the high-thermal-conductivity graphene is introduced, so that the heat conductivity of the composite material is improved; and the high-thermal-conductivity metal-based composite material can be used as a material for the thermal management of a high-power-density device.