As strongly nonlinear, dynamic, multidimensional and large scale systems, power systems in operation are always influenced by the external and internal disturbances; and thus the serious threat to stable operation of systems exists. In this paper, based on the three order mathematical model of single machine infinite systems with valve control and under the conditions that systems have unknown damping coefficient, Storage functions of control system are constructed on the basis of improved adaptive backstepping and Lyapunov methods. A nonlinear adaptive robust controller and a parameter update law are obtained. The derived controller can not only ensure that the system states are bounded and keep the nonlinearity of power systems and the real-time parameter estimation but also improved the speed of parameter identification.