Based on the differential geometric control theory， this paper proposes a novel zero dynamic design method for a class of nonlinear non-minimum phase systems by adopting dynamic feedback to the controlled systems to obtain stable zero dynamics through dimension extension. The nonlinear control law is then derived by means of the linear control design method. Furthermore， both the optimality of the control law and the closed-loop system stability are mathematically and strictly proved using the Hamilton-Jacobian-Bellman equation and the center manifold theory， respectively. The nonlinear optimal governor control law of the large-scale hydraulic turbine generating set is derived by referring to the hydraulic turbine governing system model. Simulation results show that the proposed governor control strategy for hydraulic turbines can enhance the transient stability of power systems more effectively than the conventional control law.