This paper studies the robust tracking control problem of an underactuated surface vessel with parameter uncertainties and external disturbances, and a sliding-mode methodology is proposed. The controller is divided by backstepping method into the kinetic loop and the dynamic loop. In the kinetic loop, the desired position and the current position are used to determine the reference surge and sway velocities which are employed as the virtual control law in stabilizing the position errors. In the dynamic loop, the virtual control law is considered the new tracking target, and the real control law is designed by the sliding-mode method to realize the tracking control of the reference velocities. Eventually, the tracking control for the underactuated surface vessel is achieved. Simulations of the tracking control for the underactuated surface vessel with disturbances or without disturbances are performed. Results validate the effectiveness of the proposed method.