In this paper, the trajectory tracking control problem of under-actuated surface vessels (USVs) influenced by unknown-varying environmental disturbances is studied. An adaptive disturbance observer (ADO) is incorporated to estimate the unknown frequency disturbances, then dynamic surface control technology (DSC) is introduced to solve the problem of mass computing in the designing of the virtual control law. Based on which, a tracking controller is constructed to ensure USVs to track the reference trajectories well by combining the disturbance-observer-based-control with DSC strategy. At last, the stability analysis process of USVs is provided using backstepping procedure, Simulation results illuminate the performance of this trajectory tracking method.