Abstract: Energy storage is an adjustable resource to enhance system flexibility, however, due to its cost constraint, it is difficult to allocate large amounts of energy storage in power systems. To this end, an optimal allocation method of energy storage capacity that considers flexibility and source-load uncertainty is proposed, considering the impact of thermal units and energy storage on flexibility in the system, and using fuzzy chance constraints to describe source-load uncertainty.. Based on the improved IEEE 30-node system for comparison and validation, the results show that the proposed model has more reasonable energy storage configuration results and lower system operation cost than the traditional model.