The invention provides a three-dimensional graphene / phase change energy storage composite material and a preparation method thereof. The technical scheme is as follows: graphene and a phase change energy storage material are in situ compounded, wherein porous graphene with a three-dimensional structure is used as a heat conductor and a compound die, and a solid-liquid phase change organic material is used as the energy storage material and filler. The three-dimensional porous graphene is compounded with the phase change material, the phase change energy storage material is partitioned in a plurality of pore spaces and is in tight joint with the graphene wall so as to greatly increase the effective heat contact area, and the highly linked graphene three-dimensional heat conduction network channels can realize rapid system heat exchange. In addition, due to the capillary adsorption capacity of porous graphene, the liquid-state phase change energy storage material is localized, so as to effectively prevent seepage. Therefore, the three-dimensional graphene foam has good designability, and becomes a lighter and more effective heat dissipation material for electronic devices.