Abstract: PM_2.5 samples were collected during autumn in Liaocheng City from October 15th to November 14th, 2017, and the water-soluble ions (Na⁺, ${\mathrm{NH}}_4^{+}$ , K⁺, Mg²⁺, Ca²⁺, F^-, Cl^-, ${\mathrm{NO}}_3^{-}$ and ${\mathrm{SO}}_4^{2-}$ ) were analyzed to study the pollution characteristics of PM_2.5 in autumn in Liaocheng City. The results showed that PM_2.5 mean concentration was (104.9±50.5) μg/m³ during the observation period, 0.40 times higher than the daily average concentration limit of Grade Ⅱ standard of Ambient Air Quality Standards (GB 3095-2012). The average concentration of water-soluble ions was (52.2±35.1) μg/m³, which accounted for 45.7%±11.3% of PM_2.5. ${\mathrm{SO}}_4^{2-}$ , ${\mathrm{NO}}_3^{-}$ and ${\mathrm{NH}}_4^{+}$ were the main ions, accounting for 85.2% of the water-soluble ions and 45.0% of PM_2.5. As the air pollution got serious, the concentrations of water-soluble ions increased. In addition, the proportion of ${\mathrm{NH}}_4^{+}$ increased gradually, that of ${\mathrm{SO}}_4^{2-}$ , ${\mathrm{No}}_3^{-}$ increased first but then decreased, while that of other ions especially Ca²⁺decreased, which indicated that the increase of PM_2.5 concentration was mainly affected by the secondary inorganic transformation. The average sulfur oxidation rate (SOR) and nitrogen oxidation rate (NOR) during the observation period were 0.28 and 0.27, respectively, which showed that the secondary transformation of SO₂ and NO₂ was higher in autumn and the secondary pollution was much serious. In addition, SOR and NOR were positively correlated with temperature and relative humidity, and SOR was more sensitive to relative humidity, but NOR to temperature. PM_2.5 in Liaocheng City presented weak alkaline in autumn, and ${\mathrm{NH}}_4^{+}$ were primarily in NH₄NO₃ and (NH₄)₂SO₄ forms. The results of the principal component analysis revealed that secondary inorganic transformation, dust sources and industry emissions were the main sources of water-soluble ions of PM_2.5 during autumn in Liaocheng City.