Hydrogen combustion and explosion is an important safety issue in nuclear power plants (NPPs) containment during postulated severe accidents or in utilization of hydrogen. It is significant to understand the hydrogen flow and distribution in space for mitigating hydrogen risk. In this paper, a numerical model to investigate hydrogen flow and distribution in a vessel is established using computational fluid dynamics (CFD) code GASFLOW. Hydrogen is simulated by helium which is used to study the hydrogen distribution. The k-ε turbulent model is selected to establish the numerical model, and the numerical model which has no obstacle inside the vessel or includes the obstacle inside is verified under medium momentum conditions of injected gas by comparing numerical results with experimental data. Regardless of the presence of the obstacle in the vessel, helium stratification occurs under all momentum condition of injected gas. When the obstacle is present, it blocks the flow path of the injected helium to the upper space, then the helium volume concentration in upper space is lower than the condition that there is no obstacle in the vessel. As the initial Froude number increases from 0.19 to 19.29, the flow mechanism around the obstacle under high momentum condition of injected gas is different from that under medium or low momentum conditions. Consequently, the boundary of the helium stratification moves down, and the distribution of helium looks more uniform in most area of the vessel for high momentum conditions of injected gas.