Energy Stability Control (ESC) is proposed to deal with the excitation control problem of the synchronous generator unit based on Lyapunov theorem. In order to build the relationship between the dynamic and static performances and the system stability, Lyapunov function is constructed by terminal voltage, rotor speed and active power. Considering the negative definiteness of the derivative of Lyapunov function, a completely controllable system is actively constructed to design the moving trajectory of the system along with nonlinear excitation feedback. In this condition, when matrix theory, pole arrangement method and Lyapunov theorem are used, the derivative of Lyapunov function can be negative definite for any real number except for equilibrium point. Therefore, the global asymptotic stability of the system is guaranteed. At the same time, terminal voltage regulation is achieved resulted from the energy relationship between terminal voltage and system global asymptotic stability. Finally, simulations demonstrate the effectiveness of the ESC proposed in this paper.