Abstract: The leakage phenomenon of the main steam header flange of the test steam generator during the fast-cooling test on the engineering tests facility for steam generator (ETF-SG) was analyzed using the finite element program ABAQUS. A three-dimensional finite element model of the main steam header with bolted flange connection was developed. The processes of pretension, pressurization, temperature rise and fast-cooling were simulated. Analysis results show that the main reason for leakage is the obvious increase of the sealing face opening displacement, which is caused by the deformation of flange sealing face and the weakening of bolt force during the fast-cooling process. The processes of fast-cooling under various cooling rates were simulated further, results of sealing face opening displacement indicate that the limiting the cooling rate to an appropriate extent can improve the sealing performance.