Liquid ceramic precursor was synthesized by boron trichloride and methyldichlorosilane as raw materials through the two-step reaction with hexamethyldisilazane and ammonia. The precursor was pyrolyzed and decarbonized in ammonia to produce SiBN ceramics. The ceramic yield and elemental composition of the precursor were easily regulated by changing the feeding ratio. The pyrolysis process of the precursor and the ceramic products obtained from different temperatures were characterized by NMR， FTIR， XRD， SEM and elemental analysis. The results show that the ceramization is completed at 900 ℃ under ammonia， and the boron content of the pyrolyzed product is more than 13 %（w）. The ceramic product， which is obtained at 1 400 ℃ nitrogen or air environment， remains amorphous and has high temperature and oxidation resistance.