In allusion to the low frequency oscillation existed in the AC/DC power system with VSC-HVDC system contained, the general HVDC additional damping controllers have higher order of matrix affect the practicability and availability, a kind of reduced-order controller design method based on projective control theorem is proposed to be used in the design of damping controller in VSC-HVDC transmission system. In the first, the oscillation character and transfer function are identified by the TLS-ESPRIT algorithm. Besides, the state-feedback matrix is