The invention discloses a preparation method of an S-band and C-band composite electromagnetic wave-absorbing material. The preparation method comprises the following steps: with carbonyl iron powder and nano-BaTiO3 as raw materials, sequentially performing high-energy ball milling, performing thermal surface oxidation, and mechanically mixing with nano-BaTiO3 to obtain composite powder. By the preparation method, excellent electromagnetic parameters, namely relatively high permeability, a relatively high dielectric constant real part and a relatively low dielectric constant imaginary part, can be achieved within S and C bands (2-8GHz). The composite electromagnetic wave-absorbing material provided by the invention has high wave-absorbing performance within the S and C bands, and through calculation, the measured electromagnetic parameter reaches -4 dB at 2 GHz, is smaller than -6 dB at 3-8 GHz and is smaller than -10 dB at 4-5 GHz when the thickness of the composite wave-absorbing material is 1 mm. The composite electromagnetic wave-absorbing material prepared by the preparation method provided by the invention is simple in process, can be continuously produced, is good in actual effect, and has a relatively high use value and a relatively high application prospect.