In order to achieve a balance of power supply and demand when there is high penetration of renewable energy, it is very important to increase the flexibility of the power system. This paper first analyzes the flexibility demand of renewable energy on the power supply side, grid side, and load side of China's power system. Then the technical system for improving the flexibility of renewable energy power systems and related practices and effects that have been carried out in China are summarized. Given the national energy transformation goals, the main indicators for measuring the flexibility of the power system and its medium- and long-term development goals are proposed. Finally, a development path is designed to achieve the above goals from the four aspects of flexible power supply, flexible power grid, flexible load and the digital power grid platform. This work is supported by International Cooperation and Exchange Project of National?Natural?Science?Foundation?of?China (No. 71961137004) and Project of State Grid Jiangsu Electric Power Company (No. J2019098).