Abstract: As to the low frequency oscillation of AC/DC parallel power system which contains VSC-HVDC, multi-objective control method based on linear matrix inequalities is applied in additional control of VSC-HVDC. The reduced order model of the system is identified by the least square-estimation method via rotational technique (TLS-ESPRIT). By comprehensively taking both the robustness and control cost performance of controller into account, a multi-objective function is determined, additional damping controller for H₂/H_∞ multi-objective robustness is presented, and designed traditional pole placement controller is compared. The electromagnetic transient system model for VSC-HVDC system with four machines and two areas is built in PSCAD/EMTDC, the results of the eigenvalue analysis and digital simulation of the testing system show that robust damping controller has better damping characteristics and lower control costs in such case when inner parameters of system change greatly.