A distributed energy storage system (DESS) can efficiently perform voltage management of the distribution network through four-quadrant power operation. The existing DESS optimal configuration research oriented to improve the voltage profile lacks a comprehensive consideration of the power four-quadrant operation of DESSs and the three-phase imbalance of the distribution network. Therefore, a two-stage optimal configuration method of power four-quadrant DESS suitable for an unbalanced distribution network is proposed. In the placement stage, based on the three-phase Newton-Raphson power flow algorithm, a full-dimensional voltage sensitivity analysis method is proposed to explore the optimal location of DESSs from the perspective of improving system voltage quality. In the capacity allocation stage, considering the economic benefits of system operation, the DESSs economic operation model is established with the lowest average annual operation cost and the smallest voltage deviation of buses as the optimization objectives. Then the particle swarm-grey wolf algorithm is used to solve it. Taking the improved IEEE 33-bus three-phase distribution network as an example, simulation results verify the effectiveness of the proposed method.