The load and distributed generation output in an active distribution network have obvious cyclical variation characteristics. The existing energy storage planning methods for active distribution network always output a storage configuration with fixed installation location and capacity. There is still a large space for energy storage to fully exert its flexibility and value to the distribution network. This paper proposes a planning method of dynamic energy storage configuration in an active distribution network. Through this method, the energy storage devices are dynamically allocated between different operating scenarios of the distribution network regarding varying load demands as well as distributed generation outputs. The access points, capacities and operational modes of energy storage devices in different scenarios are taken as the decision variables, the maximum comprehensive economic benefit brought by energy storage is taken as the objective, and thus the Mixed-Integer Second-Order Cone Programming (MISOCP) model for dynamic energy storage configuration is constructed. Taking the IEEE 33-node system as an example, the feasibility and effectiveness of the proposed method are verified. The proposed method can be treated as a useful expansion to the flexible operation of an energy storage device, through which the energy storage can serve well for various operating scenarios of the active distribution network and thus its value can be fully developed. This work is supported by National Key Research and Development Program of China (No. 2016YFB0900600) and Science and Technology Project of China Southern Power Grid Shenzhen Power Supply Bureau Co., Ltd. (No. 0900002019030304ZCPW00001).