In recent years, the magnetorheological (MR) fluid damper-based semiactive control systems have received considerable attention for protecting structures against natural hazards such as strong earthquakes and high winds. In this paper, an improved modal controller integrated with a Kalman-filter observer is proposed for vibration control of spatial structures. The proposed modal controller can reduce the dimension of the control equation and thus can relieve the calculation burden greatly. In the proposed control system, a Kalman-filter observer, which estimates the full controlled modal states from local accelerometer outputs, is designed for rendering the controller to be more applicable to real spatial structures with a large number of degrees of freedom (DOFs). A numerical example of a lattice roof structure installed with MR dampers is presented. The effectiveness of the controller is evaluated under both Tianjin and El Centro earthquake excitations. The simulation results demonstrate not only its effectiveness of reducing vibration of spatial structure, but also its adaptability for versatile loading conditions.