As the size of the wind turbines increases, the structural loads causing fatigue stress on mechanical components become more dominant. Torsional vibration reduction in drive-train helps to reduce the stress and avoid potential resonances with power grid system. In this study, a new approach based on disturbance observer and band-pass filter is proposed. The design of the parameters leads to solve a constrained optimization problem. The stability of the system can be guaranteed by the constraint conditions. The effectiveness of the proposed method is confirmed via simulations and experiment.