To effectively solve power fluctuation problems caused by rolling wind in the off-grid wind power system, an energy storage strategy is proposed to design a power management unit with two-parameter controller and buffer-capacitor in the windmill generator system for charging the lithium-ion battery continuously and environment-friendly. A buffer capacitor circuit has been raised to stabilize the output power of windmill generator. Besides, in accordance with the technical requirements of the battery charging characteristics, the charging mode needs to protect the lithium-ion battery to extend its life. Base on analysis methods of power electronic circuits, a power management unit has been designed. By the electrical simulation toolkit in the Matlab/SimuLink soft, it has shown that simulation results comply with the design requirements and prove that the energy storage strategy is feasible and the control effect is ideal. Therefore, this design strategy which full uses of the wind energy and improves the utilization rate of wind energy can be applied to the full wind-speed and off-grid wind power system.