Abstract: To address the frequent actuator failures caused by complex underwater environments and the inherent characteristics of Unmanned Underwater Vehicle (UUV), this research investigates the prescribed performance path-following fault-tolerant control for underactuated UUV subject to ocean current disturbances, model uncertainties, and actuator faults. For UUV safe navigation, a path-following fault-tolerant controller is designed by integrating an improved prescribed performance function and a barrier Lyapunov function to achieve full-state-constrained fault-tolerant control. A novel predefined-time disturbance observer is developed to estimate the lumped disturbances in UUV path-following, including ocean currents, parameter perturbations, unmodeled dynamics, and thrust loss due to actuator faults. The lumped uncertainties with actuator faults are incorporated into the prescribed performance fault-tolerant controller for compensation, ensuring all path-following state errors remain within predefined bounds. Simulation results demonstrate that the position error, attitude angle error, and angular velocity error converge rapidly while strictly adhering to the prescribed safety constraints, with a steady-state position error bound of 1 meter and an attitude angle error bound of 0.05 radians. When actuators experience 80% thrust loss, the disturbance observer quickly estimates the lumped disturbances, and the controller compensates within 1 second without significant path-following deviation. The maximum transient error does not exceed 20% of the prescribed limit. These findings validate the strong robustness of the proposed method against actuator faults. By unifying disturbance observation and prescribed performance constraints, the fault-tolerant control structure is simplified, achieving both fast fault response and full-state safety guarantees. This work provides a universal solution for high-reliability UUV navigation in complex environments.