During the process of long-term service, the mechanical properties of the fractured rock mass located at the periphery of concrete lining and strengthened by grouting method (grouting zone) deteriorate gradually as a result of leakage dissolution and chemical corrosion, greatly affecting the long-term service performance of the hydraulic tunnel under high external water pressure. For predicting and simulating the long-term evolution process of the service performance of the hydraulic tunnel under high external water pressure with the consideration of the deterioration of the grouting zone, the direct seepage-stress-damage-deterioration coupling model is proposed based on the effective stress principle of saturated porous medium and by taking into account the permeability variation of surrounding rock, grouting zone and lining concrete and the time-varying deterioration of the mechanical properties of grouting zone. Afterwards, the proposed model is implemented in ABAQUS, and the long-term service performance evolution process of a deep buried diversion tunnel under high external water pressure is simulated. Numerical simulation results show that the hydraulic gradient in the rock close to the excavation boundary and the external water action applied on the concrete lining decrease gradually in the process of long-term service, and the surrounding rock, grouting zone and concrete lining contract inward. As a result of grouting zone deterioration, the tunnel under high external water pressure exhibits the deformation instability mode with the sign of concrete lining’s crushing.