To study on the sealing performance of metal to metal contact bolted flanged joints under insulation,a DN600CLASS300 metal to metal contact (MMC) bolted flanged joints test device in high temperature with insulation was established,and the temperature distribution and flange rotation at 500 ℃ was measured by thermocouples.A threedimensional flange joint finite element model was developed by finite element software ANSYS to calculation the temperature distribution and flange rotation of MMC bolted flanged joints,and the simulation results were validated by experiments.The sealing performance was also analyzed under insulation.Results showed that the temperature of each component was improved when insulated.The higher the temperature,the worse the sealing performance.The decay rate of bolt load is accelerated by insulation,but the insulation has less influence on sealing performance.