The invention relates to a preparation method for a diatomaceous earth-supported nanometer carbon composite adsorption material, and belongs to the field of nonmetallic mineral material deep-processing and environmental engineering. The surface and pore passages of diatomaceous earth are taken as micro-reaction areas, glucose or starch is taken as a carbon source, and generated nanometer carbon particles are uniformly supported to the surface and the pore passages of diatomaceous earth to obtain the efficient nanometer carbon composite adsorption material with diatomaceous earth as a supporter by a hydrothermal synthesis method. According to the method, a process is simple, the nanometer carbon particles are uniformly and densely distributed on the surface and in the pore passages of diatomaceous earth, the adsorption of nanometer carbon and diatomaceous earth to pollutants in water is greatly improved, and nanometer carbon and diatomaceous earth have high potential application value in the field of wastewater treatment.