The invention provides an optimal output calculation method and an optimal output calculation device for an energy storage system for smoothening wind power fluctuation, and is applied to the technical field of wind power generation. The optimal output calculation method includes performing the Fourier transform to ultra-short period wind power prediction curves to acquire a wind power prediction curve spectrogram, determining a cut-off frequency pre-selection interval for first-order low-pass filtering, performing first-order low-pass filtering to the ultra-short period wind power prediction curves to acquire a wind energy storage fluctuation rate prediction curve and a wind energy storage power prediction curve, determining the maximum cut-off frequency meeting the preset wind energy storage fluctuation rate, determining the optimal cut-off frequency selection interval, acquiring an energy storage power curve, calculating charge and discharge efficiency of the energy storage power curve, determining the optimal cut-off frequency, determining the optimal output curve of the energy storage system, and guiding output of the energy storage system according to the optimal output curve of the energy storage system. Optimization of energy status variation of the energy storage system is taken into consideration, and thus the overall implementing process is simple and easy to control, and the optimal output calculation method and the optimal output calculation device have good practicability.