The invention discloses a high-heat-conductivity composite material and a preparation method thereof. The preparation method comprises the following steps: evenly mixing fibrous heat-conduction filler and high-heat-conductivity filler graphene in a mass ratio of 1:1-1:200 with thermoplastic polymer, dispersing for 0.5-24 hours, granulating to obtain high-heat-conductivity composite grains, and putting the grains in a die to carry out hot molding at 170-280 DEG C under the pressure of 10-18 MPa. The thermoplastic polymer is any one of polyethylene, polypropylene, polyvinylidene fluoride, polytetrafluoroethylene, polyperfluoroalkoxy ester, nylon, polymethyl methacrylate, polycarbonate and polyvinyl chloride. The fibrous heat-conduction filler is carbon fiber or carbon nanotube, and the high-heat-conductivity filler accounts for 5-35 wt% of the high-heat-conductivity composite material. The high-heat-conductivity composite material has the advantages of excellent heat-conduction property, low consumption of heat-conduction filler, favorable mechanical properties, simple preparation technique and low cost.