Abstract: To deal with the drastic fluctuation of net load caused by large amount of distributed power generation connected to distribution network, based on the combination of local restriction with quantitative evaluation of overall flexibility a method to optimally allocate the energy storage system was proposed. Firstly, the X-mean method was utilized to analyze the annual operation scenario of distribution network and through the clustering the key scenario was chosen. Secondly, the quantitative calculation model of operation flexibility in distribution network and the local flexibility constraint were established. Finally, based on the established flexibility evaluation model, a bi-layer iterative model to solve the optimal allocation of energy storage systems in distribution network was constructed. The standard IEEE-33 bus distribution network system was utilized in the design and simulation of a real distribution network. Simulation results show that the proposed flexible planning method for the allocation of energy storage systems in distribution network is effective and economic.