Objective <p>To investigate the seasonal characteristics, sources, secondary generation of water-soluble ions (WSIs) in PM_2.5 and the impacts of biomass burning (BB) on WSIs in Southwest China.</p> Methods <p>A total of 560 PM_2.5 samples were collected in three cities, with nine WSIs analyzed for concentrations. Source contributions were identified by combining positive matrix factorization (PMF) model and backward trajectory analysis. The changes of sulfur oxidation ratio (SOR), nitrogen oxidation ratio (NOR), meteorological factors and O₃ were analyzed, and the formation of secondary ions was discussed.</p> Results <p>The daily concentration of total WSIs ranged from 5.08 to 27.18 μg m⁻³, averaging 10.90 ± 3.91 μg m⁻³ and representing 29.52% of the PM_2.5 mass. The seasonal variation characteristics of PM_2.5, secondary inorganic ions (SNA) and K⁺ were consistent, and the peak concentrations appeared in spring. BB and secondary inorganic aerosols dominated WSIs sources, with Southeast Asia (SEA) identified as the primary origin area of springtime air masses. In spring, elevated O₃ levels and high ammonia emissions jointly promoted the photochemical reaction of SO₂, NO₂ and NH₃ to generate (NH₄)₂SO₄ and NH₄NO₃. The rising concentrations of K and NH resulted in the alkalinity of aerosols.</p> Conclusions <p>BB had a significant effect on WSIs in PM_2.5 in Southwest China. The focus of spring particulate matter pollution control in Southwest China is to establish an effective cross-border air pollution prevention and control system, and attention should be paid to the management of O₃ and NH₃ emissions.</p> Graphical Abstract <p></p>