Abstract: The development of energy storage technology provides an effective method for solving power quality issue caused by large scale grid-connected wind farm. Rational capacity of energy storage device can not only enhance utilization efficiency of energy storage device, but also smooth wind power output, and improve power quality. In this paper, by considering that the economical profit of wind farm for rotating backup capacity can be reduced by energy storage device and the balance of power fluctuation, multi-objective particle swarm optimization algorithm is used to optimize capacity of storage equipment for wind farm, and both the economic and stability of wind farm brought by energy storage device can be reflected by using Pareto optimal solution set. By taking classical day power output for one wind farm as simulation case, the effectiveness of proposed method is verified, and the works in this paper can provide reference applicable scheme for selecting capacity of energy storage device for wind farm.