As to the low frequency oscillation of AC/DC parallel power system which contains hybrid multi-terminal DC, a reduced-order controller design method based on the principle of projective control is introduced. Three-terminal hybrid DC transmission system is built, and the control strategy is determined. The model of the system is identified by the least square-estimation method via rotational technique (TLS-ESPRIT), a state feedback gain matrix is obtained, and dominant eigenvalue of the closed-loop system is retained, then a low-order output feedback controller is designed by projective control, and a damping controller based on linear matrix inequalities is compared. Finally, the results of the eigenvalue analysis and digital simulation of the testing system show that projective controller has lower order, stronger robustness and better damping characteristics.