Aiming at flexibility problem originated from integration of large-scale intermittent generation source, such as wind farm, photovoltaic power station, etc, the corresponding research achievements and process are systematically summarized from three aspects of power system flexibility: the concept and definition, quantitative indexes, evaluation method. On this basis, key issues of power system flexibility are expatiated and discussed, which include the generalized definition and connotation of power system flexibility; the quantitative flexibility evaluation indexes of power unit, power grid and power subsystem; the flexibility evaluation method in generation expansion planning, unit commitment and economic dispatch scenarios, etc. Finally, the research direction of power system flexibility is prospected and the concerns in the next research stage of power system flexibility evaluation are expounded.