Large scale of grid-connected renewable energy and diversified development of load have come up with higher requirements on power system flexibility. In this paper, the assessment and optimization methods on power system flexibility are reviewed, and a comprehensive flexibility optimization strategy on power system with high-percentage renewable energy is proposed. First, the definition of power system flexibility is generalized and that the cause of serious wind abandoning and solar abandoning in China that is insufficient flexibility is pointed out. Next, several flexibility assessment indexes put forward in various references are compared and two system-level flexibility quantitative indexes are emphatically introduced. In addition, power system flexibility resources are analyzed according to the hierarchy of “source-grid-load-storage”, which are also comparatively studied. Finally, starting with the perspective of planning, operation and management, a comprehensive power system flexibility optimization strategy is put forward.