This paper proposes a nonlinear excitation controller based on synergetic control theory. Firstly, the deviations of generator terminal voltage and rotor speed and active power are used to make a manifold considering the basic requirements and characteristics of excitation system in order to regulate terminal voltage of generator and suppress power oscillations. Then, the control law of synergetic excitation controller (SEC) is deduced from the nonlinear model of synchronous generator. According to the operating characteristics of power system, the principles of selecting the controller parameters are also presented. Finally, simulation results of a single machine infinite bus (SMIB) power system demonstrate that the performance of the proposed controller is better than that of the conventional AVR+PSS excitation controller both in terms of terminal voltage regulation of generator and damping power system oscillation under small as well as large disturbances.