Tunnel safety has long been a major concern for transportation and government entities. Noxious fumes, deadly gasses, groundwater seepage and flooding threats have occurred in main transportation systems. It is necessary to isolate the water inrush area as soon as possible, to gain time for further treatment. The method of stopping water using large-diameter airbags is quite efficient, and the contact area between the surface of the airbag and the tunnel inner wall will produce frictional force to resist the fluid pressure from expansion of wetting area. This paper assumes that the airbag can't be stretched, and combining with the boundary conditions in the tunnel of the airbag and the stress condition, the deformation and the force characteristics of the airbag under external pressure are studied. Formula of shape change and internal pressure increase of the airbag is obtained, and the relationship between the external and internal pressure is proposed. The control factors for leakage resistance, influence of the aspect ratio, as well as initial pressure on the shape and internal pressure were analyzed. The theoretical formula is verified via the model test, indicating good agreement.