A path tracking control algorithm based on the improved line-of-sight guidance algorithm and adaptive sliding-mode heading control algorithm is proposed. Based on the Lyapunov and cascade theory, the proposed path tracking control system is proved to be uniform globally asymptotic stable (UGAS) when the target tasks are all achieved. Compared to the traditional line-of-sight guidance algorithm, the improved guidance algorithm in the paper can realize the real-time estimation and compensation of the drift angle by introducing the adaptive observer. At the same time, the design of the forward-looking distance makes the manipulation of the underactuated USV more flexible. Based on the maneuvering model and parameters identification of the USV called "sturgeon 03", the path tracking contrast simulations were carried out on MATLAB/SIMULINK software. The simulation results show that the path tracking algorithm proposed in the paper has more dynamic performance and less steady-state error than the traditional LOS guidance algorithm. Then, the sea-field test using the USV called "sturgeon 03" confirms that, under the action of the tracking control algorithm, the USV can track the desired path accurately, and the track error is relatively small during the whole tracking.