Environmentally friendly and efficient liquid oxygen/kerosene was used as rocket propellant for the new generation of carrier rockets.The boiling point of liquid oxygen was as low as -183 ℃，which put forward strict requirements for the low-temperature sealing of the carrier rocket body valve disc.At present，the metal/non-metal（polytetrafluoroethylene） low-temperature valve disc was prepared by hot pressing process.The non-metal interior was prone to producing black impurity defects，which had the risk of seal failure.In view of the phenomenon of black impurity inside the non-metal，this paper analyzed the formation reasons of black impurity defects inside the non-metal in detail.It was proposed to reduce the black impurity phenomenon by non-metallic powder sieve process，improving the sealing reliability and safety of the valve disc.At the same time，the risk analysis of seals with black spot defects inside was carried out through mechanical test and valve assembly test.The black impurity inside the non-metal is confirmed by element analysis that the black impurity is caused by the oxidation of low molecular weight polytetrafluoroethylene under high temperature and high pressure.In addition，the mechanical test results show that the black spot inside the non-metal has no effect on its tensile strength，modulus，elongation at break and impact strength.The low-temperature valve assembly test with black spots inside the sealing surface ±1.5 mm also passes the benchmark test.