Abstract: In order to understand the influence of NH₃ and aerosol liquid water content (AWC) under different pollution levels in winter on the formation of water-soluble secondary ions in PM_2.5, the concentrations of particulate matter, secondary ions and precursors (SO₂, NO₂ and NH₃) in Baoding City were analyzed. The ISORROPIA-Ⅱ was used to calculate the AWC and pH in PM_2.5. The results showed that: (1) The concentrations of PM_2.5, SO₂, NO₂, and NH₃ during the heavy pollution (AQI>200) in Baoding City were 3.0, 1.1, 1.3 and 0.8 times than those during non-pollution period (AQI < 100) in winter from 2017 to 2018. The secondary formation of gaseous precursors was one of the important reasons for this pollution formation process. During the heavy pollution period, NO₃^- concentration showed the largest increase (1.3 times than corresponding average concentration), followed by NH₄⁺(1.2 times), and SO₄^2- (0.9 times). (2) The excess NH₃ index (0.1) indicated that the atmosphere of Baoding City was under ammonia-rich condition, and the formation of NO₃^- was mainly limited by HNO₃. (3) The concentration of AWC in PM_2.5 during the heavy pollution period reached 93.6 μg/m³, which is 20.6 times that of non-pollution period. During the observation period, the secondary formation of SO₄^2- in Baoding was mainly caused by the oxidation of SO₂ by NO₂ and NH₃ in the liquid phase on the particle surface. The secondary formation of NO₃^- included the heterogeneous transformation of NH₃ and the heterogeneous hydrolysis of N₂O₅. (4) The strongest sensitivity of pH was observed after controlling TA (total ammonia), followed by TS (total sulfur), TN (total nitrogen), and TA+TS+TN (synchronously change the concentration of total ammonia, total sulfur, total nitrogen). As the pollution intensified, the pH sensitivity of TS and TA decreased, while pH sensitivity of TN increased. The sensitivity of AWC is weak when the concentration of TS, TN and TA is changed separately, but stronger when the concentration of TS, TN and TA is changed simultaneously. The change of AWC is closely related to the concentration of secondary ions. The study shows that the formation of SNA in winter pollution period in Baoding City is dominated by the liquid phase oxidation of water, and NH₃ can maintain the oxidation process of high-pH particulate matter.