Wind power integration will influence the existing low-frequency oscillation modes and possibly introduce new modes. Suppression of one single low-frequency oscillation mode may decrease damping of other modes. Based on detailed modeling to the doubly-fed induction generator (DFIG), the state space equations of wind power system are established and the control variables and output variables are determined. The impact of DFIG output is analyzed to reveal the necessity of the regional pole placement of the weakly damped oscillation modes. A reduced-order model is derived considering both the existing and newly introduced modes. The regional pole placement based on linear matrix inequality is applied to design the output feedback controller. All the weakly damped modes are placed within the desired region to increase the damping ratio and improve the stability margin. Numerical results show that the designed output feedback controller effectively increases the damping of all weak damping modes and suppresses the oscillation characteristic of wind power systems. This work is supported by National Natural Science Foundation of China (No. 51637004).