Power fluctuation caused by wind speed variations may be harmful for the stability of the power system as well as the reliability of the wind power converter, since it may induce thermal excursions in the solder joints of the power modules. Using the wind turbine rotor inertia energy for power leveling has been studied before, but no quantified analysis or generic design method have been found. In this paper, the transfer functions from the wind speed to electrical power, electromagnetic torque, and rotating speed are built based on which the rotating speed controller is designed in the frequency domain for power leveling. Moreover, the impact of other parameters on power leveling, including the time constant of maximum power point tracking (MPPT) and the rotor inertia, are also studied. With the proposed optimal design, the power fluctuations are mitigated as much as possible, while the stability of the rotating speed is still guaranteed. Moreover, the oscillation of the electromagnetic torque is also reduced, and the performance of the MPPT is only weakened slightly.