Abstract: In view of problem that traditional PID control for synchronous generator cannot meet with requirements of oscillation suppression and improvement of dynamic stability limit of power system, the paper proposed the optimal excitation control method for synchronous generator based on AC tracking. It gave deviation and linearization model of excitation system of synchronous generator based on Lagrange method, analyzed definition method of the optimal excitation control quantity and built simulation model of synchronous generator system based on the control method. The simulation result showed that the control method can real-timely control excitation system of synchronous generator to ensure output stability of synchronous generator comparing with traditional PID control method.