The invention discloses a preparation method of a diamond reinforced silicon carbide (SiC) composite wafer, and belongs to the field of semiconductor material preparation. The method comprises the following steps: plating a 5-20nm SiNx thin layer on a SiC carbon polar surface subjected to vacuum heat treatment at 800-1000 DEG C; and after microwave hydrogen plasma treatment is carried out for 1-5 min, methane with the hydrogen flow proportion being 5-10% is introduced, the operation lasts for 5-15 min, and diamond high-density nucleation and C-Si bond formation are achieved based on nitrogen atom escape and carbon atom permeation. And then, silane with the hydrogen flow proportion being 0.1-1% is introduced while methane is reduced to 3%-5% for growing diamond, the silane flow is slowly reduced within 10-30 min till closing, and the SiC/diamond composite gradient transition layer is deposited. And then repeating the circulation process of introducing nitrogen with the hydrogen flow proportion of 1-5% every 5-20 min in diamond growth and keeping for 1-5 min, closing the nitrogen and methane after the diamond reaches a certain thickness, slowly cooling to 600 DEG C or below in hydrogen plasma, then raising the temperature to 800-1000 DEG C, treating for 0.5-5 h, and slowly cooling, and finally, the diamond/SiC composite wafer material with low stress and strong combination is realized by polishing the diamond surface.