This paper studies the heat conduction limit of fire protection for isolated rubber, and provides theoretical evidence and experimental data for fire protection. Based on one-dimensional unsteady heat conduction, formula under ISO834 standard fire is derived. According to the GB9978.1-2008 fire test, three kinds of components, LNR500, LNR1500 support board fire protection, LNR500 support board with flexible fireproof material fire protection, are tested.Modeling and finite element thermal analysis are carried out on ABAQUS. The numerical analysis and fire test results verified our theory. Based on the derivation and experiment data, when the temperature of the support surface reaches the critical temperature 150 C, the table of the limit thermal conductivity of fireproofing board with different thickness can be referred in engineering applications.