Abstract: The hydrogen mitigation system is widely used in nuclear power plant for the purpose of ensuring the integrity of containment under severe accidents. The ignitor and the recombiner in hydrogen mitigation system were analyzed using 3D CFD code GASFLOW, and their characters were summarized. The ignitor can effectively reduce large amount of hydrogen by igniting the flammable cloud, and can increase the average pressure and the temperature in containment obviously. The recombiner needs long time running to eliminate large amounts of hydrogen and would not cause obvious increase of the pressure and the temperature. It is possible to reduce much hydrogen without causing deflagration or detonation in large area if the location and starting time of the ignitor are reasonable.