Hydrogen-steam gas mixture may be injected into containment with flow regime varying both spatiallyand transiently due to wall effect and pressure difference between primary loop and containment insevere accidents induced by loss of coolant accident. Preliminary CFD analysis is conducted to gain informationabout the helium flow regime transition process from jet to buoyancy plume for forthcomingexperimental study. Physical models of impinging jet and wall condensation are validated using separatedeffect experimental data, firstly. Then helium transportation is analyzed with the effect of jetmomentum, buoyancy and wall cooling discussed. Result shows that helium distribution is totallydominated by impinging jet in the beginning, high concentration appears near gas source and wallwhere jet momentum is strong. With the jet weakening, stable light gas layer without recirculating eddyis established by buoyancy. Transient reversed helium distribution appears due to natural convectionresulted from wall cooling, which delays the stratification. It is necessary to concern about hydrogenaccumulation in lower space under the containment external cooling strategy. From the perspective ofexperiment design, measurement point should be set at the height of connecting pipe and near the wallfor stratification stability criterion and impinging jet modelling validation.