The invention provides a multi-objective optimization planning method of an energy storage system. The multi-objective optimization planning method comprises the steps of obtaining a generating parameter and a load parameter of a power transmission network within preset time and dividing the preset time into a plurality of time intervals with equal time duration; carrying out flexibility analysis on the power transmission network within the plurality of time intervals in sequence; building a single-interval energy storage device optimization scheme set; correcting the power transmission network according to the single-interval energy storage device optimization scheme set; and carrying out power flow multi-interval optimization on the corrected power transmission network within the preset time to obtain an optimal configuration scheme of an energy storage device. Compared with the prior art, the multi-objective optimization planning method of the energy storage system has the advantages that configured time intervals, bus node, power value, electric energy value and optimal cost value of the energy storage device can be simultaneously determined through flexibility analysis, power flow single-interval optimization and power flow multi-interval optimization on the power transmission network, so that the problem of network congestion of the power transmission network can be effectively solved.