Abstract: In this paper,the design principle and its actual operation of external steam cooler of No.3 high pressure heater of steam turbine generator set in a 1 000 MW power unit are mainly introduced,and its thermal economy is analyzed. Besides,the shunt coefficient and temperature rise of the steam cooler of the unit are studied. The difference between the unit performance test data and the design value is compared and the causes of the difference are analyzed from the perspective of design and equipment failure.