Abstract: For a 660MW ultra-supercritical double reheat power generation unit, a high-temperature molten salt energy storage system was proposed to store the high-temperature steam extraction at the primary and secondary reheater cold sections and release the thermal energy through a high temperature feed water and molten salt heater in order to improve the flexibility of the double reheat power generation unit. Simulation on the high-temperature molten salt thermal energy storage system during thermal energy storage from steam extraction and release of molten salt thermal energy to the feed water heater is conducted to present the load response and thermal performance characteristics in the double reheat boiler and the steam turbine under 100% THA. The maximum peaking output could reach at 45MW during the thermal storage process and the corresponding load time is shortened by 600s; the maximum peaking output could reach at 12MW during the thermal release process and the corresponding load time is shortened by 1800s, which can effectively improve the load response of the double reheat power generation unit. The ramp rate reaches at 6.82%Pe during thermal energy charging process and 1.82% Pe during the thermal discharging process at 100% THA, which significantly improves the load responding to power grid. The research results show that the high-temperature molten salt thermal storage system can improve the flexibility of double reheat power generation units.