The invention relates to a thermal conductive insulation plastic with low viscosity and high thermal conductivity and a preparation method thereof, and belongs to the technical field of thermal conductive insulation plastic preparation. The thermal conductive insulation plastic is composed of the following components in parts by weight: 100 parts of polymer resin, 60 to 180 parts of inorganic powder, 15 to 30 parts of chemically bonded graphene/carbon nanotube multi-arm thermal conductive filling material, 1 to 3 parts of coupling agent, 0.2 to 10 parts of lubricant, and 0.1 to 0.2 part of antioxidant. The preparation method comprises the following steps: pre-drying polymer resin, inorganic powder, and chemically bonded graphene/carbon nanotube multi-arm thermal conductive filling material, stirring all raw materials in a high speed stirring machine for 1 to 1.5 hours so as to evenly mix all raw material; transferring the mixture to a double screw granulator, extruding and granulating the mixture, and after granulating, drying the granules to obtain finished product of thermal conductive insulation plastic. The provided preparation method can obtain an insulation material with high thermal conductivity, at the same time, the original mechanical properties can be maintained, moreover, the preparation technology is simple, the equipment requirement is low, the conditions can be easily controlled, the cost is low, and the method is suitable for industrial production.