Abstract: For Suzhou-Nantong GIL power gallery tunnel, the prototype tests using the straight assembling segmental lining method are carried out. From the aspect of water pressures on the force, deformation and crack resistance of the segmental structure, the influences of water pressures on the mechanical properties of the segmental structure under the straight joint assembling condition are studied. The results show that: (1) The straight assembling segmental lining has a large displacement at the position of dome, and increasing the water pressures can effectively control the dome deformation and the overall elliptical deformation of the segmental structure. However, the maximum single point displacement of the segment is easier to reach the limit than the ellipticity. (2) It is recommended to take the single-point maximum deformation rate of 2‰~2.5‰ as the deformation control standard. (3) Under the effects of high water pressures, the joint opening of the segmental structure mainly occurs near the K-block, which is caused by dense joints, the minimum structural rigidity and the maximum deformation. The increase in the water pressures has a significant limiting effect on the longitudinal joint opening, and can reduce the force of the corresponding connecting bolts. (4) Increasing the water pressures will greatly improve the crack resistance of the segmental structure and reduce the tensile stress of the main reinforcement of the segmental structure, but it will also increase its compressive stress and hoop stress. (5) The increase in the water pressures improves the mechanical properties of the segment structure to a certain extent. However, the high water pressures cause the segmental structure to be in a state of high axial compression, which is prone to crushing and shearing at the joint and the damage is sudden. The research results have important guiding significance for the design of underwater shield tunnels.