Abstract: The regional integrated energy system (RIES) integrates and coordinates diversified and flexible resources to maintain local multi-energy supply-demand balance, optimize the adjustable capacity, realize cross-regional energy complementarity, and improve energy utilization efficiency by sharing the adjustable capacity. It plays an essential role in deeply mining the adjustable potential of massive users and giving full play to the value of system flexibility. First, this paper analyzes the driving force and barriers to building RIES under the aggregation of diversified and flexible resources and systematically reviews the existing research. Then, based on the current research clues, the critical problems of the RIES are analyzed from three aspects: in-depth mining of the spatio-temporal dynamic evolution of the functional regulation capacity of the RIES, optimization of the functional regulation capacity of complex nonlinear dynamic RIES, and optimal coordination of multiple RIES clusters. On this basis, the research framework of the RIES under the aggregation of diversified and flexible resources is proposed from three key issues that jointly affect the stable and economical operation, namely, the assessment of the functional regulation capability, the autonomous coordination and optimization, and the collaborative cluster control of the RIES. Finally, the critical research contents are analyzed and elaborated.