Abstract: In order to reduce the probability that the air-cooled island unit of the thermal power plant in north China will be frozen due to the influence of low temperature in cold winter and ensure the safe operation of the power plant throughout the year,protective devices are added to the air-cooled island unit of the thermal power plant. Based on the characteristics of large steam extraction heat supply flow and long time of air cooling island unit,in order to avoid the problem of back pressure rise caused by the installation of anti freezing device, an automatic plugging device for air cooling island of large heat supply unit is designed.The power system, transmission system, traction system and cover system of the device are introduced, and its working principle is analyzed. The project implementation effect test and economic energy saving effect test were implemented about the automatic plugging device in the the air cooling island of a 2×310 MW power plant phase I project of Guoshen group.The results showed that the mechanical control is realized through the operation of the motor control device, which reduces the freezing risk of the air cooling module of the thermal power plant, ensures the smooth operation of the air cooling island throughout the year, effectively reduces the number of air coolers put into operation, reduces the power consumption rate, and has remarkable economic effect. It has certain reference and guiding significance.