To investigate the characteristics of water-soluble inorganic ions (WSIIs) and their roles in the occurrence of fine particulate matter (PM_2.5) pollution events, we collected 23-h integrated PM_2.5 samples over 4 months in 2021 in Mianyang, Southwest China. Nine inorganic ions (Na⁺, NH₄⁺, K⁺, Mg²⁺, Ca²⁺, F⁻, Cl⁻, SO₄²⁻, and NO₃⁻) were analyzed through ion chromatography; hourly SO₂, NO₂, and O₃ concentrations and meteorological parameters were measured using online monitoring instruments. Annual mean concentration of WSIIs was 11.7±9.1 μg m⁻³, constituting 35.0% of PM_2.5. SO₄²⁻, NO₃⁻, and NH₄⁺ were the dominant ions. NO₃⁻ concentration was highest in winter and lowest in summer; by contrast, SO₄²⁻ concentrations were comparable in winter and spring but lower in autumn. NH₄⁺ concentration followed the same seasonal pattern as the sum of SO₄²⁻ and NO₃⁻ concentrations. PM_2.5 pollution events primarily occurred in winter, with the mean concentration of 107.4±12.0 μg m⁻³ on pollution days. SO₄²⁻, NO₃⁻, and NH₄⁺ concentrations were 1.6-2.2 times higher on pollution days than on non-pollution days, but SO₂ and NO₂ concentrations increased by a factor of approximately 1.2, implying the enhanced secondary inorganic ions formation. PM_2.5 pollution event occurrence was mainly associated with low wind speeds, high aerosol pH, and high aerosol water content. These findings highlight the key features of WSIIs during pollution periods in medium-sized cities and provide basic data for evaluating the effects of clean-air policies on the reduction of PM_2.5 concentrations.