Abstract: Objectives The trajectory tracking problem of underactuated unmanned surface vehicles (USVs) under multiple constraints, including unknown system dynamics, control input saturation, and obstacle avoidance, is addressed in this paper. A fixed-time trajectory tracking control method based on an event-triggered mechanism is proposed. Methods First, a coordinate transformation is applied to handle the underactuated nature of the vehicle. A fixed-time disturbance observer is designed to estimate and compensate for composite disturbances caused by model uncertainties and external interference. An artificial potential function is employed to construct a guidance law that satisfies obstacle avoidance constraints. An auxiliary system is introduced to compensate for performance degradation due to input saturation. Finally, a relative-threshold event-triggered mechanism is incorporated to reduce the frequency of control signal updates, thereby decreasing actuator actions. Results Based on Lyapunov fixed-time stability theory, it is proven that the trajectory tracking error converges within a fixed time, while safe navigation is ensured under obstacle avoidance constraints, and Zeno behavior is avoided. The effectiveness and safety of the proposed trajectory tracking control method are validated through MATLAB simulations. Conclusions Composite disturbances can be accurately estimated, and performance degradation caused by input saturation can be effectively compensated using the proposed method. Control signal frequency is reduced and obstacle avoidance is achieved, providing a reliable technical solution for trajectory tracking control of underactuated unmanned surface vehicles in complex environments.