In order to facilitate the design and analysis of the tracking controller, coordinate transformation was introduced to establish a typical underactuated USV mathematical model. The Serret-Frenet coordinate system was established to describe the tracking error variables. Then, considering the underactuated system, the updated law of the tracking parameters was taken as the additional input of control, the tracking control of the USV position was realized by using the improved line-of-sight guidance algorithm. In view of the disturbance such as ocean current and so on, the adaptive sliding mode technique was used to design the heading and speed tracking control law, respectively, which realizes the stabilization of heading error and longitudinal velocity tracking error. Based on Lyapunov and cascade theory, the uniformly global asymptotic and semiglobal exponential stability of the tracking control system was proved. The simulation results show that the proposed control algorithm can deal with the disturbance such as ocean currents to a certain extent, and the tracking effect is good. Moreover, it overcomes the continuous excitation of angular velocity and can track the straight and curved line at the same time.