In order to suppress the low frequency oscillation caused by the interaction of AC and DC system, a strategy based on sensitivity theory of single HVDC system is proposed. The key point of this strategy is using Prony algorithm to identify the critical oscillation modes and applying TLS-ESPRIT algorithm to choose the generator with high participation of system’s critical oscillation modes. By testing the rotor speed change of the important generator under different DC modulation, the most efficient DC line in damping critical oscillation mode can be found out. Simulation with small mode DC modulation controller for a multi-terminal HVDC system shows its effectiveness.