Abstract: The hydrogen igniter combustion mitigation system is widely used in nuclear power plant for the purpose of ensuring the integrity of containment under severe accident. Combined with the numerical simulation using GASFLOW and other test data, the results obtained from the hydrogen combustion test carried out in a 20 m³ cylinder with 9.28% hydrogen were analyzed. The results show that the hydrogen is completely consumed, leading to a rapid increase in temperature and pressure inside the tank. The heat could be transferred from the gas to the cylinder by three means: radiation, convection and phase change. This heat transfer process could largely reduce the temperature and pressure in the cylinder, resulting in cooling of the gas and depressurization of the tank. At the beginning of the combustion, the flame will propagate upward along the centerline of the cylinder as a result of buoyancy; once reaching the roof of the tank, it turns around and spreads downwards. The flame propagation speed is around 11.15 m/s. Under the influence of different internal structures in the test, the propagation of the flame is more complex.