Abstract: An underwater tunnel, whether in the construction or operation phase, is always located under the water, with an unlimited water supply and constant head; thus, the water pressure is one of the main loads to be considered for the lining structure design for an underwater tunnel. In this paper, the characteristics of water pressure on composite linings in underwater tunnels are studied by a theoretical analysis, model test, and field-measured data analysis of the Xiamen subsea tunnel. The research show that: (1) when only considering tunnel drainage, the grouting circle can reduce the water pressure on the lining. Water pressure on the lining reduces with an increase of grouting thickness, and the better the grouting effect is, the more obvious the reduction in water pressure, with the higher water pressure shared by the relevant grouting circle. However, the unlimited increase of the thickness of the grouting circle has little effect on the reduction of the water pressure, so a reasonable grouting thickness is 3~8m; (2) the water pressure on the lining obviously increases with the strengthening of primary support permeability resistance. Certain water pressure on the primary support must be considered in the design, and the water pressure on the primary support is mainly dependent on the ratio of the permeability coefficient of the grouting layer to that of the primary support; and (3) the reduction coefficient of water pressure on the secondary lining depends on the discharge of water permeating into the primary lining, namely, the ratio of the water drainage of the tunnel to the volume of water permeating into the primary lining, regardless of the grouting effect and the permeability coefficient of the primary support and secondary lining. During the determination of water pressure on the secondary lining, the impact of the initial seepage field has to be considered. This research can provide a reference for the structural design of underwater tunnels and mountain tunnels with high water pressure.