For the karst tunnel with pipeline cavities in the surrounding rock, it is very easy to accumulate high water pressure in the pipeline cavity due to the influences of heavy rainfall on the surface and groundwater, which will lead to lining cracking, water leakage, and water gushing diseases. To explore the influence of high water pressure on the lining structure, model tests of pipeline karst tunnels under high water pressure were carried out, and the internal force characteristics of tunnel lining under the influence of cavity position and water head height were studied. Based on the test findings, the numerical calculation models of extended working conditions were established. The effects of cavity diameter, cavity position, and water head height on the lining structure were further explored. The results show that the inner side of the lining in contact with the cavities bears a larger positive bending moment when there are karst cavities around the tunnel, and is the most unfavorable position of the lining to bear loads. The internal forces of the lining increase greatly, with the increase of cavity diameter and water head height in the cavity. The location of the cavity affects the internal force distributions of the lining, and the water pressure resistance capacity of the lining is the smallest when the cavity is located at the tunnel vault. The research results can provide a reference for the structural design and safe construction of pipeline karst tunnels.