Abstract: Integrated energy system（IES） flexibility can serve the improvement of power system adjustability under high proportion of renewable energy.However,the mechanism of IES flexibility is complex and its connotation is rich,there is still lack of unified understanding and systematic elaboration.Firstly,the research status of power system flexibility and IES flexibility at home and abroad is comprehensively reviewed from the perspectives of flexibility-oriented planning and design,operation and regulation,market mechanism,and quantification and evaluation methods.The connection and difference between power system flexibility and IES flexibility are then summarized and analyzed.Secondly,the necessity of studying IES flexibility in the context of the high proportion of renewable energy is pointed out,and the basic connotation of IES flexibility is revealed from the perspectives of definition,basic elements,and measurement system.Thirdly,the general form of the mathematical model of IES flexibility is defined,and a specific elaboration is given with examples.Furthermore,the research framework of IES flexibility is constructed from three aspects:physical mechanism,enhancement approaches,modeling,quantification and evaluation methods.Finally,the key problems of IES flexibility research in terms of connotation,mechanism,and enhancement approaches are prospected.