This paper presents a linear state-space equation for a single machine-infinite system based on Park model, which is used for the design of optimal excitation control law. Applying this equation on the basis of optimal control theory, a new excitation controller is designed, in which the terminal voltage, active power, reactive power, angular velocity and angle are the feedback. Load angle and reactive power are introduced to excitation control law, which are closely related to stability and quality. At the same time, in order to improve the terminal voltage regulation accuracy, the integral control is introduced to the excitation control law. The result of MATLAB simulation indicates that compared with other methods, the proposed excitation control can effectively improve the power system’s stability in large and small disturbance, as well as voltage regulation precision.