Abstract: During the severe accident of nuclear power plant, the thermal-hydraulic character of hydrogen in the containment is very complicated. The flow and distribution of hydrogen are influenced by many factors, such as the access of the containment, the production rate of hydrogen, the steam fraction in the source mixture, and so on. The paper studies the hydrogen distribution in the containment using the CFD code—CFX, and focuses attention on the steam fraction in the source mixture effect on the hydrogen distribution. The results indicate that the more steam in the source mixture carries the more hydrogen away from the breach location, and improves the air circulation in the containment, so the hydrogen molar fraction near the breach location is lower, but also well-proportional in other areas.