In allusion to the phenomena of inter-area low frequency oscillation in the big interconnected power system, also, high order of the general supplementary excitation controllers and unsatisfactory control effects exist, a modified projective controller on the basis of projective control is designed and used in the control of supplementary excitation. The oscillation modals are identified by the TLS-ESPRIT algorithm firstly; according to the pole-placement theory, the state feedback matrix of the system is identified; then retaining r order dominant eigenvalue on the basis of the projective control with p order eigenvalue retained; by regulating the adjustable matrix P0 to control the eigenvalue bounds of interference matrix to reduce the influence of original oscillation mode to the system. Simulations of three-phase grounding, one-phase grounding faults and different time delays are done to the system in a model of four machine two domains system without additional controller, with controller based on state observer and modified projective controller in PSCAD/EMTDC. The results validate that the system controlled by the modified projective controller has better damping character and dynamic performance, fewer effects caused by different time delays, and better engineering practice.