Abstract: The reheated steam temperature of the ultra-supercritical unit is higher than the superheated steam temperature, which causes the fourth-stage extraction steam to present the characteristics of high temperature and low pressure. The addition of the No. 4 high-pressure external steam cooler can increase the final feed water temperature of the boiler, reduce the first-stage extraction steam and improve the thermal efficiency of the unit. In this paper, we established the model of a 660 MW ultra-supercritical unit’s heat recovery system to analyze the influence of the external steam cooler on the thermal efficiency of the unit under different operating conditions. The results show that by adding external steam cooler, the final temperature of boiler feed water under the design working condition and the actual working condition of the power plant can be increased by about 4.2 ℃ and 3.7 ℃ respectively. Under different working condition the average reduction of heat consumption of the unit is about 19 kJ/kW·h, the reduction of standard coal consumption for power generation of the unit is about 0.698 g/kW·h, and the efficiency of the steam turbine is increased by about 0.11%. The energy-saving effect of the external steam cooler shows fluctuation of a “V” shape as the load of the unit increases. And the lower the load of the unit, the more effective the external steam cooler will be. The research results can provide technical support for the variable-condition operation of the unit and the technical renovation of adding an external steam cooler to a similar unit.