A new reduced-order controller design method based on improved projective control is introduced. And the low-frequency oscillations can be suppressed by high voltage direct current additional controller through this method. The system reduced mode and relative oscillation frequencies are obtained through TLS-ESPRIT algorithm. Then the controller is designed by improved projective control, after the pole-placement method is applied to obtain a state-feedback matrix. The traditional pole-placement controller based on observer is compared with the projective controller at the same time. The advantages of lower order, stronger robustness and better control effect have been proved through simulation.