Abstract: During severe accident in nuclear power plant, the hydrogen-air combustion is a main risk threatening the integrity of the reactor containment. However, the existence of steam in hydrogen-air mixture can cause some differences on consequence of combustion. CAST3M software was used to simulate hydrogen-air and steam-hydrogen-air combustion in small and medium scale geometry. Firstly, a typical test case in THAI facility was simulated to indicate that the relevant combustion model was applicable. Then, a cylinder with the diameter of 2.2 m and the height of 6 m was used as combustion chamber. The hydrogen volume fractions of 8%, 10% and 12% in the hydrogen-air mixture case were calculated respectively, and the results with the same hydrogen fraction and the steam volume fraction of 25% in the steam-hydrogen-air mixture case were compared. By analyzing the results of temperature, pressure and flame speed with and without steam in the combustion chamber, the steam in the hydrogen-air-steam mixture leads the peak pressure, the peak temperature and the flame propagation speed to reduce significantly. According to this simulation, steam can suppress hydrogen-air combustion and reduce the combustion consequence.