Abstract: In a flexible power generation environment, boilers in coal power plants are often operated at low loads, exacerbating the air preheater clogging condition. The differential pressure between the inlet and outlet of the flue gas side of the air preheater is an important indicator for the evaluation of air preheater clogging. By analysing the historical operation data of DCS of a 660MW coal-fired unit, the hierarchical analysis method （AHP） and the method of approximating ideal solution ranking （TOPSIS） are used to comprehensively evaluate the degree of air preheater clogging. The study shows that the degree of air preheater clogging is closely related to the unit load, the probability of ammonium bisulfate generation increases under high load, and the tendency of ash accumulation in the air preheater is particularly serious when the Radian number distribution representing ammonium bisulfate deposition is above 10,000, accounting for about 30.3% of the total. After the unit was shut down for maintenance operation, the Radian number of more than 5000 basically did not occur under low and medium load conditions. The degree of clogging improved significantly after maintenance. Comprehensive evaluation shows that the clogging of the air preheater after maintenance has been improved, but the degree of clogging still shows a slow rising trend over time, and measures such as regular ash cleaning and adjustment of operating parameters need to be taken to control the generation of ammonium bisulfate. This model can effectively evaluate the clogging status of the air preheater.