The invention relates to a method for preparing a SiBCN-containing thermally-polymerisable ceramic precursor. The method comprises the steps of enabling vinyl dichlorosilane and hexamethyldisilazane, which are in a certain ratio, to react at a certain temperature, adding a certain amount of NaBH4 reducer into the reaction product for reduction after reaction is complete, then, adding dimethyl vinyl chlorosilane into the reduced reaction product to continue reaction, heating up the reaction product to distill off a solvent and some unreacted raw material molecules, finally, adding a solvent n-hexane into the reaction product to wash the reaction product, thereby obtaining a colorless viscous liquid product, i.e., the soluble and fusible SiBCN ceramic precursor. According to the synthesis method, the operation is simple and convenient, experimental conditions are easily controlled, experimental steps are reduced, the cost is low, the prepared SiBCN ceramic precursor is soluble and fusible, has good processability, is good in fluidity and can be subjected to thermal polymerization directly, the yield of the prepared product can reach 60% the input amount of chlorosilane, and the yield of sintered ceramics can reach 64%.