Abstract: The integration of new energy units and flexible resources has changed the form and characteristics of traditional distribution systems.Flexibility is the primary indicator that new distribution systems need to pay attention to.As the complexity of the system increases, the difficulty of evaluating system flexibility also increases, which is not conducive to subsequent power grid planning and scheduling work.Supply and demand balance is one of the important theories for system flexibility evaluation, which has the potential to be applied to the flexibility evaluation of complex distribution systems.However, there are still doubts and a lack of systematic discussion in related research.It provides a review of the flexibility supply and demand balance and its evaluation.Firstly, it summarizes the flexibility of the power system and the flexibility supply and demand balance, and provides the basic assumptions for the theoretical existence.Secondly, the classification of flexible resources is based on the supply and demand balance, and future research directions for flexible resources are prospected.Finally, it analyzes the advantages and disadvantages of existing flexibility evaluation indicators, which provide reference for subsequent research.