High–pressure methane gas generally exists stably under methane hydrate stability zone at several hundred meters cutting through the marine sedimentary strata. The usually employed bottom simulating reflector (BSR) for hydrate recognition represents the interface between hydrate and fluid areas in typical natural methane hydrate reservoir system with hydrate, water and gas layers. In this study, the gas–seawater migration in hydrate reservoir was simulated through gas–seawater injection, and the existence of hydrate–containing sealing layer was experimentally confirmed. The hydrate reformation was observed by MRI during the gas–water injection process above the methane hydrate phase equilibrium pressure and it is the fundamental reason that hydrate reservoir has sealing effect on free gas. As the decrease of pore spaces in sediments, the interaction of seawater and hydrate in the reservoir products capillary sealing in the narrow space, thus the free gas and seawater migration are inhibited and the free gas exited stably underlying the hydrate layer. However, low methane concentration in seawater caused by high gas–water flow rate (4–1 ml/min) resulted in the hydrate dissociation, the hydrate–bearing sediments can’t produce the sealing effect. Hydrate further forms in the sealing layer and leads to seawater depletion until it is too salty to form hydrate. Finally, the gas layer, water layer and hydrate layer coexist under the seabed. In addition, the hydrate–containing sealing layer could be broken through, and the breakthrough pressure is a significant parameter for hydrate reservoir.