The energy storage system makes randomly fluctuating wind power feasible for pre-determined power dispatching. To reduce impacts of wind power random fluctuation component on power grid, an optimization method for determining storage capacity is proposed to smooth wind power minute-scale, based on the statistical analysis of wind power minute-scale wave characteristic. Considering the wind power minute-scale characteristic, the optimization model is built to confirm storage capacity and maximum charging and discharging power by using interval theory of probability and statistics. The study can smooth wind power minute-scale by small capacity storage and reduce the impacts of wind power on power grid, and the model is proved to be effective.