Abstract: Against the background of new power systems and next-generation coal-fired power upgrading, ensuring the safe operation and improving the operating economy of deep peak-shaving units are of great importance. To investigate ash deposition, blockage, and corrosion in air preheaters and to reveal the formation mechanisms of deposits at different elevations, a four-sector rotary air preheater in a 2×660 MW coal-fired power plant was selected as the research object. XRD, XRF, SEM-EDS mapping, and XPS were used to characterize the phase composition, micro-morphology, elemental distribution, and surface chemical states of deposits collected from the middle and bottom sections. The heat-transfer model was established to evaluate the condensation risk. The results show that the mass fractions of Fe2O3 in the middle and bottom deposits were 51.66% and 42.68%, while those of SiO2 were 24.00% and 29.98%, respectively. The SO3 content in the bottom deposits was 9.19%, approximately 3.9 times that in the middle deposits (2.35%). The middle deposits mainly exhibited a composite deposition structure composed of fly ash particles and iron oxides, whereas the bottom deposits formed a composite fouling layer consisting of ammonium salt crystals, corrosion products, and fly ash particles. Cold-end blockage of the air preheater was attributed to the coupled effects of ammonium salt deposition, fly ash capture, and low-temperature corrosion. With 100%, 75%, 50%, and 40% turbine heat acceptance (THA) used as boundary conditions, the model showed that reducing the primary air flow rate from 100% to 70% of the baseline value had only a limited effect. In contrast, when the primary air flow rate was maintained at 80% of the baseline value, a moderate reduction in the secondary air flow rate significantly increased the outlet flue gas temperature and shifted the condensation zone downward while narrowing its range. Therefore, mitigation strategies should integrate temperature elevation, coordinated regulation of air flow rates, and soot blowing into periodic operation management, so as to reduce continuous cementation by viscous ammonium salts and damage to heat-transfer elements caused by repeated thermal shock.