There is an unavoidable time-delay involved when wide area signals are transmitted through a long distance. To solve this problem， based upon unified Smith predictor（USP）， a novel method for the additional excitation controller designing is presented. This method eliminates the impact of time-delay by using USP at design stage. The standard H ∞mixed sensitivity approach， which is derived by solving linear matrix inequalities with additional pole-placement constraints， is employed to design the damping controller. The predictor based method proposed is applied for the damping controller design for a two-area power system. Simulation results show that controller could improve the damping ratio of inter-area mode effectively， even though the feedback signals arrive at the controller site after a finite time-delay.This work is supported by the Specialized Research Fund for the Doctoral Program of High Education（No. 20050213006） and the Key Science Research Project of Heilongjiang Province（No. GD07A304）.