The inertia variation of the AC servo system highly influences the servo performance. In order to improve the dynamic performance of the servo system, it’s necessary to identify the system inertia and to auto tune servo controller at real time. Online inertia identification method based on MRAS was introduced. A MRAS method with dynamic adaptive gain and filter time was presented to solve the contradiction between identification speed and accuracy. The symmetrical optimum method was used to tune the servo control loop automatically with the identified inertia. The consistent and robust performance of servo controller was obtained in this way. Simulation and experiment results showed the effectiveness of the proposed method.