Using PhBr and BF₃: OEt₂ as the raw materials, the poly(p-phenylene borane) (PPB) is synthesized, After PPB is pyrolyzed in high temperature, the product containing 54.4% B₄C. 32% free canbon and 13.6% B₂O₃ is obtained. If the carbon fiber(Cf) is dipped in the solution of PPB and pyrolyzed in high temperature, the Cf coated by B₄C (B₄C-Cf) is obtained, Comparing with raw Cf in same treat conditions, the tempereture of thermal oxidation resistance of B₄C-Cf has increased by 200℃. The δ₆ of Cf/SiC matrix composites made by using B₄C-Cf 1800℃ is improved 10.5% and the KIC is improved 19.4%. At 1850℃, the δ_ь is improved 31.6% and the KIC is improved 5.8%.