A field study was conducted to investigate the deposit formation of the low-pressure economizer (LPE) in a 330 MW thermal power plant. The deposits on the LPE were inspected visually, and eight deposit samples were collected at the inlet and outlet of the LPE according to the site conditions. The samples were characterized by X-ray fluorescence, X-ray diffraction, scanning electron microscopy with energy-dispersive spectroscopy, and particle size analysis. The results showed that the thick deposits on the supporting tubes at the LPE inlet were mainly due to the ash particles that had absorbed the H2SO4 condensate in the flue gas before the LPE inlet. NH4HSO4 began to form on the heat-exchange tubes at the LPE inlet. Both the H2SO4 condensation and NH4HSO4 formation stimulated the deposit formation on the heat -exchange tubes at the LPE inlet. The plugging of the heat -exchange tubes at the LPE outlet was primarily induced by the condensation of much more H2SO4. H2SO4 and NH4HSO4 could also react with the fly ash to form sulfates. The LPE deposition mechanism was discussed, and the coupling model of the H2SO4, NH4HSO4, and fly ash behaviors during the deposit formation under different temperature conditions was propokd.