The robust H∞ control problem for the generator excitation system with the damping coefficient uncertainty and external disturbances, is addressed. Storage functions of the control system are constructed based on modified adaptive backstepping sliding mode control method and Lyapunov method. A nonlinear robust H∞ controller and a parameter updating law are obtained simultaneously. The controller can not only attenuate the influences of external disturbances on the system output, but also has strong robustness for system parameters variation. Since the controller design is based completely on the nonlinear dynamic system without any linearization, the nonlinear property of the dynamic system is well preserved. The simulation results show that more rapid speed response and stronger robustness can be obtained by the proposed method than the conventional adaptive backstepping and adaptive backstepping sliding mode control methods.