Modal control is very important for effective operations of active optical systems. Several types of modes are studied for a 1.2 m active mirror, such as Zernike modes, free vibration modes of the primary mirror and eigen modes. Eigen modes are referred to modal analyses of membrane deformable mirrors and modal control based on eigen modes of the primary mirror haven’t been found yet. Considering the fitting accuracy of the active mirror for various modes and the relationship between fitting capability and the number of modes adopted, defocus, 3rd coma and 9 eigen modes are used for modal control of the 1.2 m active mirror. Defocus and 3rd coma are corrected through the movement and tilt of the secondary mirror and 9 eigen modes are corrected through force actuators at the back of the primary mirror.