Abstract: To realize the segmented monitoring of ash fouling respectively in hot and cold section of a tri-sectional rotary air heater, a segmented model was set up based on the heat transfer model of air heater and by defining the utilization coefficient as the cleanliness factor, of which the calculation steps were determined according to the heat balance principle and the real-time operating data. Taking the tri-sectional rotary air heater in a 1 000 MW ultra supercritical once-through boiler as an example, variation trends of cleanliness factor in both the hot and cold section were analyzed during soot formation and blowing period. Results show that the variation trends of segmented cleanliness factor calculated by the model can reflect actual fouling situation in hot and cold section during stable load operation of the air heater; however, in variable load operation, the data may have some deviations, which would hardly bring problems, since soot-blowing is rarely performed under varying load conditions. The segmented model may therefore serve as a reference for ash fouling monitoring in hot and cold section of tri-sectional rotary air heaters.