The paper investigates the robust H_∞ problem for a class of generalized forced Hamilton system with uncertainty. We begin with presenting a design approach of robust H_∞ controller and show that the L₂ gain from the disturbance input to the regulation output signal may be reduced to any given level provided that a kind of algebraic inequality has a solution. Then, by means of the proposed method, a Hamiltonian systems-like model with uncertainties is firstly presented, which can describe the power system dynamics on a full scale, and consequently a decentralized nonlinear robust H_∞ control law is achieved by construction of a Hamiltonian function for the multimachine power system. Simulations performed on a 6-machine system verified that the proposed excitation control could adapt to the conditions under large disturbance and enhance greatly the transient stability of power system compared to other types of controllers.