Abstract: To improve the robustness of course control of the studied USV (unmanned surface vehicle) system under diverse sea conditions and settle the coupling between the surge speed and the course, a PID (proportional-integral-derivative) control algorithm whose control parameters are adaptively adjusted on-line is developed. The control parameters of the proposed PID control algorithm are some functions which are adaptively adjusted according to the surge speeds and the state errors of course. It is shown by Lyapunov function method that the proposed algorithm can keep the USV system stable. Some sea trials demonstrate that the control parameters are self-adjusted on-line according to surge speed and the state error of course under different sea conditions, moreover, the robustness and anti-disturbance performance under different sea conditions are improved, and the overshoot is decreased to less than 1.5% without static errors.