Optimal torque (OT) control is a widely used method for maximum power point tracking (MPPT) due to its simplicity. In order to overcome the adverse impacts of turbulent wind speed variations on MPPT, in several methods, modification factors have been proposed to dynamically modify the ideal power curve for OT control. However, this paper finds that the update cycles used in existing methods to adjust power curve modification factors are very long and hence these factors are difficult to be updated in a timely manner along with the wind speed variations. This thereby may deteriorate the effectiveness of wind energy extraction. Therefore, an optimal decreased torque gain (DTG) control method is proposed in this paper. Based on the persistence method, an offline mapping from the wind speed and rotor speed to optimal modification factors is established via optimal control theory. The power curve can be periodically modified online through the mapping relationship. In this method, the update cycles for these power curve modification factors are shortened from tens of minutes to seconds. The simulations and experiments show that the proposed method is more efficient than others in terms of energy extraction under varying wind speeds, especially for turbulent wind cases.