Abstract: Water conveyance tunnels are prone to issues such as concrete creep and steel corrosion during their service life. However, research on the bearing capacity and deformation characteristics of triple-composite lining structures in long-term service conditions remains insufficient. This paper conducts three-dimensional finite element numerical simulations to thoroughly investigate the structural mechanical response characteristics of triple-composite lining water conveyance tunnels under high internal water pressure and long-term service. The results indicate that the load transfer path exhibits a significant water pressure threshold effect under high internal pressure. When the water pressure is below 0.5 MPa, the bolt stress increases slowly, and the steel lining bears most of the water pressure. However, when the water pressure exceeds this threshold, the bolt stress increases rapidly, and the segment's share of the water pressure rises accordingly. In long-term service simulations, the average stress of the reinforcement increases year by year, while the circumferential strain of the self-compacting concrete (SCC) and segment concrete gradually increases without cracking. The maximum bolt stress slightly decreases due to the relaxation caused by concrete creep. Under extreme overpressure conditions (1.8 MPa), noticeable plastic damage occurs at the crown and invert of the SCC and segment concrete, suggesting that targeted reinforcement measures should be implemented for these critical areas. The findings of this study can provide a reference for the long-term safety performance prediction of other triple-composite lining tunnels.