Based on Hamiltonian energy theory, this paper proposes a water-gate and excitation controller for dual-excited synchronous generator driven by hydroturbine, such that both the rotor angle and voltage can achieve their stability. The system consists of a hydraulic turbine and a synchronous generator and is described by a sixth-order model. Through Hamiltonian energy design techniques, the system can be made asymptotically stable in the absence of disturbances, whereas the finite-gain .5% stability can be achieved in the presence of disturbances. The Hamiltonian energy approach provides us a physical insight and gives a new way to nonlinear control design. A simulation example is illustrated to show that the proposed method is effective to power systems.