This paper investigates trajectory tracking control problem for an underactuated unmanned surface vehicle under external disturbance and model parameters uncertainty. Within the framework of backstepping control, a trajectory tracking control method based on constant bearing guidance is proposed, which can avoid the singularity that often appears in circular motion of the vehicles by redefining the differential of virtual heading angle. Moreover, disturbance observers are designed to estimate the equivalent disturbance, so that the vehicle can track the desired trajectory stably in the unknown ocean environment. Finally, due to the limitation of the propellers’ mechanical performance, dynamic saturation of surge force and yaw moment is designed according to the upper and lower limits of two propellers’ thrust to ensure the stable operation of the control system. Based on Lyapunov stability theory, the stability of the system is proved. And the numerical simulation results show the effectiveness of the presented control method.