The present invention relates to a thermal structure composite material long-service-life composite coating and a preparation method thereof. The preparation method comprises: carrying out thermal insulation on a thermal structure composite material for a first preset time at a first preset temperature, carrying out pre-oxidation, and naturally cooling the thermal structure composite material to a room temperature; under a protection atmosphere, embedding the thermal structure composite material into a first embedding powder material, and carrying out thermal insulation for a second preset time at a second preset temperature to obtain a thermal structure composite material having a SiC coating; and embedding the thermal structure composite material having the SiC coating into a second embedding powder material, placing in a protection atmosphere, heating at a preset heating rate to achieve a third preset temperature, and carrying out for a third preset time. According to the present invention, with the preparation method, the product has advantages of long service life, excellent oxidation resistance, simple preparation process, convenient operation, easily available raw material, and low preparation cost.