The invention provides a power determining method for a battery energy storage system stabilizing the wind power grid connection power fluctuation. The method comprises the step one of determining a control period and obtaining the power output range of the power of a wind storage grid connection system within the next control period; the step two of obtaining the power output range, used for the smooth wind power, of the energy storage system; the step three of obtaining the actual energy storage power output range; the step four of determining the optimum energy storage power output set value within the actual energy storage power output range. The method solves the problem of set value lagging caused by the inertia time constant of a low-pass filter and improves the efficiency of stabilizing the wind power fluctuation of energy storage.