The invention relates to a preparation method for a diamond composite material. A method for preparing a ultrahigh-hardness diamond composite material by adopting discharge plasma sintering is characterized by comprising the following steps: (1) modifying a powder surface, wherein a silicon carbide film with thickness being 0.1-50 nanometers is deposited and coated on the surface of the diamond powder; (2) drying; (3) adding a sintering aid: mixing the sintering aid with an amorphous-state silicon dioxide powder, and grinding to obtain composite powder; (4) discharge plasma sintering: carrying out discharge plasma sintering for 10-30 minutes at a temperature of 1400-1700 DEG C under sintering pressure of 30-100MPa, thereby obtaining a composite material; and (5) de-molding and polishing the composite material to obtain the ultrahigh-hardness diamond composite material. The obtained high-hardness diamond composite material has the best hardness which can be up to 36 GPa. According to the method, the sintering pressure is low, and 100 MPa sintering pressure accounts for 2 percent of normal diamond preparation pressure (higher than 5GPa). The diamond composite material prepared by the method is high in compactness and is high in hardness.