After the high-pressure heater of steam turbine is out of service, the feed water temperature will drop. In order to maintain the evaporation capacity, the combustion must be strengthened to increase the temperature of hearth. After the fume temperature at hearth outlet is increased, the fume amount and flow rate through the superheater and reheater will be quickly increased, at this moment, if the unit is operating with rated load, the hot load of the boiler is operating under overload, thus the steam temperature is caused to rise, the pipe wall is overtemperature, the heating surface wear is aggravated and the equipment is damaged.