Abstract: With increasing installed capacity of the renewable energy, the coal-fired power plant needs to change from main power generation mode to flexible operation mode to increase the grid space for the renewable energy power generation. To improve the flexible operation of coal-fired power unit, a method to realize flexible operation of the coal-fired power plant by using main steam extraction and thermal energy storage was proposed in this paper. The exergy analysis method was adopted to analyze the three working conditions of 600 MW subcritical coal-fired power plant in the condition of 50%THA, 35%THA and 35%THA reduced by using thermal energy storage technology. Results show that the boiler is the device with the biggest exergy loss under the three operating conditions. The useful energy efficiency is reduced with the load reduction operation. When the unit load is reduced to 35% THA by adopting thermal energy storage technology, the useful energy efficiency will be reduced by 0.59% compared with the theoretical value of 35% THA. Due to the throttling effect of throttle valve and the heat transfer temperature difference between heat storage material and steam (pressured water) in the process of heat storage and release, the exergy loss is increased by 2.21% under the condition of 35%THA reduced by using thermal energy storage technology. This paper can provide a reference for the design of operational flexibility of coal-fired power plant.