The invention relates to a method for online removing 
refinery plant FCC device 
waste heat boiler 
ammonium bisulfate scale. In terms of the 
flue gas flowing direction, a 
combustion chamber, a high-temperature 
superheating section, a low-temperature 
superheating section, a high-temperature 
evaporation section, an SCR denitration section, a low-temperature 
evaporation section, a high-temperature 
economizer and a low-temperature 
economizer are arranged in a 
waste heat boiler in sequence. Two ways of high-temperature 
flue gas are led out from the rear portion of the 
combustion chamber of the 
waste heat boiler, wherein the first way of the high-temperature 
flue gas is mixed with 
flue gas after the high-temperature 
evaporation section to enable the mixed 
flue gas to reach the temperature of 350-400 DEG C, generating of 
ammonium bisulfate is reduced, and then the mixed 
flue gas enters the SCR denitration section to be subjected to denitration treatment; and the other way of the high-temperature flue gas is mixed with flue gas after the low-temperature evaporation section to enable the mixed flue gas to reach the temperature of 400-470 DEG C, and then the mixed flue gas enters the high-temperature 
economizer to enable the 
ammonium bisulfate condensed on a furnace 
pipe of the economizer to be gasified and decomposed, so that the purpose of removing the scale is achieved. The economizer of the FCC device waste heat boiler is effectively prevented from being blocked by the 
ammonium bisulfate, the increased pressure drop, caused by 
ammonium bisulfate blocking of the economizer, of the waste heat boiler is lowered, the purpose of efficient ash removing is achieved, and additionally normal operation of the boiler is not affected.