In this paper, a backstepping-sliding mode tracking control strategy based on a new preset performance function is proposed for the trajectory tracking error constraint problem of the unmanned surface vessels (USV) under unknown disturbances. By constructing a new preset performance function, not only the upper bound of the state constraint is considered, but also the convergence time of the control system can be preset by adjusting the control parameters of the system. And a disturbance observer is designed to compensate the external environment interference. The fast power reaching law is used to eliminate the sliding mode chattering and enhance the robustness of the system. Finally, the stability of the closed-loop signal is proved based on Lyapunov stability theory. Simulation experiment shows that the USV with proposed controller can track the desired trajectory in a predefined time.