Abstract: Under the "Dual-Carbon" target in China, the flexible resource regulation and utilization in power and energy systems has become an important means to reduce carbon emissions and improve energy efficiency. With the development of the distributed new energy generation technology and the demand response technology, the energy system of urban building clusters contains massive resources with flexible and adjustable characteristics. This paper takes the massive heterogeneous distributed flexible resources in the energy system of urban building clusters as the research object. Firstly, this paper introduces the quantitative analysis methods of different types of flexible resources and the characteristics of energy system flexibility. Subsequently, the relationship between flexibility aggregation and decomposition of energy systems in building clusters is analyzed, and commonly used flexibility aggregation methods are summarized. Then, aiming at the optimization of flexibility resource regulation, this paper analyzes the corresponding flexibility regulation technique for the cost target, the flexibility target, and the new energy consumption target, respectively. Finally, the application prospects in the flexibility of the energy system of urban building clusters and the problems that need to be addressed in practical applications are discussed, and the development trend of related technologies is put forward.