The invention provides an in-situ copolymerization precursor conversion method for preparing an MC-BN ultrahigh-temperature ceramic material. Vinyl monomers with inorganic elements are mixed with tri-acetenyl borazine monomers, in-situ copolymerization is conducted, precursors are prepared, and the required ceramic material is prepared by sintering the obtained precursors. The precursors obtained through the method have good performance and are simple in method and high in production efficiency.