Abstract: The interactions of the generator excitation and HVDC additional control may affect the system damping performance. The model which contains n generators and 1 DC line is derived and the wide area coordination control law of generator excitation system and HVDC additional control is also obtained. The wide-area coordinating controller (WACC) applies remote signals to compensate the interactions between HVDC and generator variables. However, the time delays introduced by remote measurement and remote control signals will affect the performance of the WACC. In this paper, the impact of time delay on system oscillation damping is analyzed, and the unified smith prediction approach is used to preserve power system performance with large time delays. The simulation on AC/DC four-machine two-regional power system is used to illustrate the influence of time delay on mode damping and the significance of delay compensation. The simulation results show that the introduce of unified smith predictor has better damping effect when generator speeds and the transmission power of AC transmission line have bigger time delay, and has certain robustness.