The primary path to realizing the “double carbon” is constructing a clean, low-carbon, safe, and efficient energy system. New energy will become the mainstay of the power system. The volatility and intermittency of new energy generation require the flexible resources of the power system to support it. Coal-fired units are the primary flexible resources suitable for China’s current and future national conditions. Flexible retrofitting of coal-fired units should solve problems such as boiler’s low-load stable combustion, water-cooled wall hydrodynamic safety, localized over-temperature, SCR inlet flue gas temperature, turbine’s thermal stress of high-temperature pressurized parts, and regular commissioning of the automatic control under the peaking. Coal-fired power units need to enhance their load ramp-up capability to meet the flexibility requirements of the power grid. Flexibility retrofit of thermal power units should be based on each unit’s actual operating condition, respecting the equipment’s operation law, and selecting a reasonable technical retrofit program based on safety and benefit.