With the large-scale development of intermittent renewable energy sources represented by wind and solar energy. The demand for power system flexibility has been significantly increased. In this paper, power system flexibility assessment is taken as the research topic. Firstly, the quantitative assessment method of flexibility is investigated. Secondly, a comprehensive assessment method of flexibility of cogeneration units based on hierarchical analysis-entropy method is proposed. Finally, the flexibility of the power system is analysed and illustrated from different perspectives. Useful information references are provided for the deployment of system flexibility retrofit.