Abstract: A trust-region based instantaneous optimal semi-active control algorithm for magnetorheological (MR) dampers is proposed in this paper. Rather than taking the active optimal control force as the target control force, the proposed control algorithm sets up an instantaneous performance index for semi-active control system. In terms of the characteristics of MR dampers, the maximum and minimum control forces for the next time step are obtained from MR damper forward model and considered as constraints for the semi-active control system. Then, by applying precise integration method, the semi-active control problem is described as a typical single target optimization problem subjected to a bound constraint. Trust-region method is then applied to find the optimal semi-active control force within the bound constraint at each time step. Numerical simulation is conducted on a typical three-story frame subjected to excitations of Tianjin wave and El-Centro wave. During the simulation, the responses of an uncontrolled structure, a structure with Clipped-optimal controller and a structure with the proposed controller are calculated and compared. Great control efforts are obtained and demonstrate the effectiveness of the proposed semi-active controller. The superiority of the controller during multi-dampers situation is further illustrated by comparing the control effectiveness with that of traditional Clipped-optimal controller, which is more suitable to semi-active control for large complex structures.