A new route to synthesize SiCN(O) precursor is adopted with vinyl trichlorosilane and solid ammonia carbamate as starting materials. The reaction is controlled in autoclave. A mixture of solid oxygen-rich polysilazane and ammonia chloride is obtained for the reason of dimensional poly-condensation. It is adopted to take the byproduct apart using chemical vapor transportation. The mixture is pyrolyzed at 1100°C for 2 h in Ar, and black SiCN(O) powder is separated from the byproduct. The chemical component of the powder is Si1.00C2.07N1.09O0.29. XRD analysis indicates that the black powder keeps amorphous at a temperature up to 1450°C. At 1500°C, SiC phase is first precipitated. With the temperature further increased, Si2N2O phase appeared. It provides a way to separate other nitrides from ammonia chloride in similar system.