The invention relates to a preparation method of a C/SiBNC composite material, which comprises the following steps: (1) polymerizing BSZ (borosilazane) monomers to obtain a polyborosilazane precursor polymer; (2) impregnating carbon fiber cloth into the polyborosilazane precursor polymer in a hot pressing mode; (3) carrying out cross-linking treatment on the carbon fiber cloth impregnated with the polyborosilazane precursor polymer obtained in the step (2); (4) carrying out hot pressing on the carbon fiber cloth obtained in the step (3), and cooling to room temperature while gradually relieving the pressure, thereby obtaining a hot-pressed infusible sample; and (5) carrying out high-temperature sintering on the hot-pressed infusible sample obtained in the step (4), thereby obtaining the C/SiBNC composite material. The preparation method is simple, and has the advantages of low cost and low facility request; and the C/SiBNC composite material has the advantages of high density, small bubbles, uniform composition, no cracking and excellent performance.