Abstract: Water conveyance tunnels are susceptible to concrete creep and steel bar corrosion during long-term service, which can degrade the bearing capacity of lining structures. However, the structural mechanical response and deformation characteristics of triple-composite lining systems under long-term service conditions remain poorly understood. Three-dimensional finite element simulations are conducted to investigate the bearing performance of triple-composite lining water conveyance tunnels under high internal pressure and long-term service. The results show that the load transfer path of the composite lining structure exhibits a distinct water pressure threshold effect under high internal water pressure. Below 0.5 MPa, bolt stress increases slowly, with most water pressure borne by the steel lining. Above this threshold (0.5 MPa), bolt stress rises rapidly, and the segment's contribution to bearing water pressure increases. In long-term simulations, the average stress of reinforcement increases over time, while the circumferential strains of self-compacting concrete and segment concrete increase without cracking. The maximum bolt stress decreases slightly due to concrete creep relaxation. Under extreme overpressure (1.8 MPa), significant plastic damage occurs at the crown and invert of self-compacting concrete and segment concrete. The findings offer insights for predicting the long-term safety performance of triple-composite lining tunnels.